IDEAS TO let simmer: Supermarkets are moving strongly into the deli and carryout realm as well as into fancy foods. Why don't they fire up a stockpot so that home cooks could buy a quart of fresh stock rather than having to resort at the last minute to canned stock or bouillon cubes? Bopes and trimmings could be forever simmering, as the market's waste became the home cook's bonanza for making soups, sauces, whatever could be improved with some real home-style broth.

And while we are serving up suggestions, we have long been waiting for someone to set up a commercial kitchen where small-scale caterers would rent space by the hour so they could bake specialties for restaurants or parties in a legally inspected location. That way homemade specialties could be prepared for public sale without running afoul of legal restrictions that cooks can't afford to comply with for just a batch of strudel or a few dozen bors d'oeuyres.

Homemade being the theme these days, it was only a matter of time before it led past homemade ice cream to homemade cones for ice cream. Where? Georgetown, of course. Right before your very eyes, freshly made ice cream cones, at Cone E. Island, on M Street between 28th and 29th. They taste pretty good, these cones, not as sweet as packaged sugar cones but certainly as crisp, with a scent like coconut. They roll into a much wider cone than usual, and so the shop sells them filled with 2 scoops of ice cream (made in Lancaster, Pa., in a dairy whose name they insist on keeping a secret) and sundae topping, whipped cream and cherry. You eat them with a spoon, at least until you get halfway down. Nice walkway hot fudge sundae for $1.40, though it could use better whipped cream, and the ice cream is rather bland. What we like best about this homemade cone business is its possibilities: Imagine 31 flavors of cone to mix and match with 31 flavors of ice cream. What we like least is the thick wad of cone that forms at the point; what used to be the best bite of an ice cream cone becomes a mouthful of cone chewing gum. The bonus is watching passersby marvel to learn that cones are made, not born.

And speaking of homemade specialties, it took us merely a few bites to grow sweet on handdipped chocolates made by a Suitland, Md., confectioner, Jon's Candy Kitchen (420-1245). They are made only to order, for $8 a pound, with centers of nuts, chocolate mousse, maple, peanut butter or mint, and can be covered with milk, dark or white chocolate or butterscotch. You can ever order them pink, green or yellow. The first to disappear from our batch, however, were the white chocolates studded with bits of nuts and perhaps some coconut, filled with a smooth and oozing chocolate mousse. Not that we had any trouble polishing off the butterscotch or the chocolate turtles. And we have yet to try a more delightful after-dinner mint than Jon's mildly minted chocolate wrapped around that buttery chocolate mousse.

Before you wash your hands of cooking fish because it lingers in your kitchen long after your summer supper, see what Cannon's has to say: Refresh your kitchen with a couple of tablespoons of vinegar in boiling water while you are cooking fish, and never let the fat in which the fish is cooking to reach the smoking point, for it is the smoke that carries the odors into the air. These tips come straight from a little blue sheet Cannon's has composed for the public; a green one is devoted to a nutritional description of seafoods. Others will include basic cooking tips, recipes, advice on how to store fish.Several a month are planned, and you can pick them up free at the store. In case that's not on your route, we'll pass on their most intriguing tip to-date. To wash your handss of fish odors, rub them with lemon juice, vinegar or toothpaste. Just think how many people Cannon's is reminding to brush after every meal.

The most satisfying diet idea in recent memory comes from a report in the Journal of the American Dietetic Association that dieters who ate soup as part of their meals lost weight more quickly than a control group who didn't include soup. It makes sense, since soup is a food that is slowly consumed, and eating slowly is one behavorial approach to dieting that has seemed effective. In addition, soup is less dense calorically than solid food. The study found that it didn't matter what kind of soup people consumed, they lost weight slightly -- but consistently -- faster. One assumes, however, it would be necessary to spoon it rather than drink it. With, we might suggest, a small spoon.

Soup companies, it seems, can't keep themselves buy enough making soups. Knorr is now meddling in the desert market, manufacturing a strawberry mousse dry mix that restaurants can whip into a mousse in six minutes flat with only milk added. It is another of those modern glories that lets a restaurant whip a busboy into a chef in only six minutes flat.

One of the few things in this world we thought didn't need to be new and improved was James Beard. Nevertheless, Knopf publishers are bringing out in the fall "The New James Beard," a book which they are calling Beard's "crowning work." And just as every recipe and menu we read is boasting dishes made "from scratch," Beard, too, is advertising that his book has been written "from scratch." We're clearing a spot as big as a nouvelle cuisine dinner plate on our bedside table and planning a two-week soup diet to get ready for this one.