It is an undisputed truism of cooking that one of the best ways to cut costs is with a knife: thinner slices, smaller pieces and the same amount goes further.

The average American homemaker is aware of this. She nods her head in agreement when a well-meaning home economist or food editor mentions it while ticking off ways to fight food price inflation. But she isn't handy with a knife and, surveys show, isn't likely to get more involved in butchery than slicing a cucumber or cutting up a child's portion of meat.

That doesn't bother Bill Broscovak. With an enthusiasm that perhaps only a son of a butcher could muster, he tours the country for the American Sheep Producers Council extolling the virtues of lamb and home butchery. Broscovak's message is that Americans should leanr how to cut, cook and serve more than just the prime cuts of lamb. (The same holds true for beef and pork, of course, but Broscovak is paid to talk about lamb.)

The industry is halfway through a rebuilding program, he reported. With present supply "40 to 50 percent behind demand," there is a sharp increase of ewes on farms and the promise of increasing the size of lambs by 25 percent with "no loss of tenderness," Broscovak forsees no loss of price either. Lamb will continue to be expensive.

Lamb accounts for a scant 1 percent of America's meat diet. The percentage doesn't hold true in fancy French retaurtants or ven in beef houses or hotel restaurants where rack of lamb or "spring" lamb chops are offered. They sell a lot of lamb. Why people eat lamb when dining out and not at home is a mystery. Another mystery is why some people will eat lamb chops - even rare lamb chops - and refuse to eat meat from any other part of the animal.

Broscovak is trying to solve these mysteries. He wants Americans to know that "spring" lamb is a thing of the past; that lamb cuts of similar size and quality are available all year round. Mutton is all but extinct in retail stores, too. Today almost all strong-flavored, 2-year-old lamb goes into soups, sausage and baby food. Because its fat mels and burns easily, the industry recommends that lamb be cooked at a low temperature - from 275 to 325 degrees for roasts. Only meat from the foreshank is tough enough to require being braised or cooked in liquid.

"The biggest problem," said Broscovak, "is overcooking. Lamb can be served rare. It should be moist and juicy."

The first time I met him, in Los Angeles, he butchered a leg of lamb - on radio! Your basic butcher might have read from a script, but not Bill Broscovak. He went to work on a tiny cutting board in that tiny studio and gave out with the entusiasm of a commentator covering a major sports event.

He doesn't promise miracles. To do the job properly you will need two special tools (a 5-inch boning knife and an 8-inch breaking knife), plus a sharpening steel and probably even a small saw. He warns that to save money (an estimated $200 to $400 per year), you will have to spend time. It can take quite a bit of practice before your skills develop. What you gain, he thinks, is control over the meat. You can portion out a cut of meat into shapes and sizes that fit family or individual needs. You can trim unhealthy excess fat and keep pieces of valuable meat that might be discarded in a supermarket.

As examples, he performed surgery on two separate shoulders of lamb. The shoulder cut is the top of the foreleg to the neck and includes five ribs. It has a complex bone structure.Last week shoulder roast with bone in cost $1.79 per pound at a local supermarket chain. The shoulder lamb chops Broscovak cut from that roast cost $1.99 a pound if sold separately, and lamb stew meat was $2.29.

Broscovak transformed one shoulder into a boneless rolled roast. First he cut behind the line of ribs, leaving a little meat, then sawed them off at the base. To ready them for the barbecue grill, he cut off the excess fat, which could drip into the coals, cause a flareup, and give the meat a burnt-tallow taste. Next he cut around the triangular shoulder blade to the roudn knuckle.

"Feel the bone with the point of the knife," Broscovak advised. "Make short strokes; don't make gashes."

Following the flat of the shoulder blade, and peeling back the meat, he extracted the knuckle and top of the foreleg. Some more pulling ("It does take some physical strength," he acknowledged) and scraping and the blade itself came away. These bones should be wrapped, labeled, frozen and used for soups or to flavor stews, he recommended.

"Be sure to follow the natural seams of the meat," he continued as he cut along the muscles to separate out the solid, elastic-like piece of prescapular fat.

The meat, 3 1/2 of the original 5 pounds, was ready to be stuffed (Broscovak suggested spinach, or rice would do nicely), rolled and tied. That, too, is a trick. Once it was shaped, he began with a firm lengthwise tie of heavy twine (another necessary investment for the home butcher) and then tied off a series of lasso circles around the roast.

From the second shoulder, he cut two 1 1/2-ince steaks across the top of the foreleg. With the fat trimmed, these would be broiled or grilled.

Next, he cut off the rib bones, sawing down through them and cutting under with the boning knife. "When you do this yourself," he said, "you can leave as much meat as you want to on the ribs."

The third operation was to cut out the chine bone with his knife and remove the "Saratoga roll" from the inside of the shoulder. "It's not sold in the retail stores I visited," he said, "but it is as tender as a rib eye." Once the fat was cut away, he sliced the minature roast into five, 3/4-ince boneless chops.

Meat from the outside of the shoulder was cut into cubes. They could be used for lamb fondue, marinated for shish kebab or ground. The smaller scraps could be ground as well, or used in a soup of stew.

Is a Boy Scout (or Girl Scout) merit badge in lamb butchery worth the effort?

For most people, probably not.

The Lobel Brothers, in their book "All About Meat," warn:

"You may tackle anything you like, but not with our blessing. Boning uncooked meats is difficult for nonprofessionals; it takes a long time and may leave your meat unrefrigerated for too long; if you are not experienced, the meat may be shredded or torn. Ask your butcher to bone the meat for your special occasions; he will be glad to do it if you ask him in time, and he will do it in seconds or minutes and give you a handsome piece of meat..."

The Lobel Brothers are butchers. They deal with the carriage trade and perform a lot of services (for a lot of money) that your friendly or not-so-friendly supermarket butcher may not have the time or skill to do.

Also, even if you can't bear to cut paper, a knowledge of meat cuts and butchering techniques can help you be a more effective shopper. The Sheep Council has commissioned three booklets, all available from Patio Lamb, 200 Clayton St., Denver, Colo. 80206. One deals with boning a leg of lamb, another with lamb cooking basics and the third with "patio cooking." The National Livestock and Meat Board, 36 South Wabash Ave., Chicago, Ill. 60603, also offers consumer information about cuts of meat - beef, port and lamb - and their uses. Details on butchering lamb are contained in USDA Home and Garden Bulletin No. 196, available from the Office of Governmental and Public Affairs, USDA, Room 506A, Washington, D.C. 20250. CAPTION: Pictures 1 through 6, Bill Broscovak turns a lamb shoulder into a rolled roast by first removing the ribs, then extracting the bones and shaping and tying the meat with twine; photos by Gerald Martineau - The Washington Post; Picture 7, no caption, By Gerald Martineau - The Washington Post