This week's look at what's new, bountiful or mysterious in the produce aisle.

With all due respect to Mr. and Mrs. Potato Head, in the root vegetable world external beauty, or the lack thereof, is not an indication of the good flavor within. Consider, for example, the unsightly celeriac (pronounced seh-LER-e-ak). This cousin of better known celery is one ugly customer. It's a gnarled, knobby and bulbous look-alike for, perhaps, a teenager from another galaxy with a common adolescent skin problem. And for that reason, celeriac, also known as celery knob and celery root--often mistaken as the root of crunchy, green stalk celery--is overlooked in the supermarket vegetable section.

Northern European shoppers, on the other hand, know that once this root has been properly pared of its outer fibers, roots and lumps, the creamy-white flesh inside is crisp and smooth-textured. It's fragrant. Celeriac is mild, with a pleasant flavor of parsley and celery. It's a common first course in a French bistro, shredded and dressed with a tart mustard and mayonnaise dressing. Like most root vegetables the seasonal peak is in the fall, but celeriac is available year-round.

HOW TO BUY THEM

Celeriac ranges in size from that of an average apple to plump grapefruit. Choose medium-size, firm roots that are free of deep dents and soft spots. Small, heavy roots are also favored for their tenderness, although once peeled there is little left to shred. Large roots tend to be woody and dry. Choose carefully. The rough exterior easily hides dark areas of decay. We found the freshest specimens we have ever encountered, with bits of green stalk still attached, at the Sunday morning American Farmland Trust FreshFarm market at Dupont Circle.

HOW TO STORE THEM

Wrap in a plastic bag. They will keep for a week or more in the coolest section of the refrigerator.

HOW TO USE THEM

Celery root is always peeled before use. One easy way to tackle the beast: slice off the top and bottom; then, pare away the lumpy skin on the sides. To prevent discoloration, briefly submerge the peeled root in water acidulated with a little lemon juice.

In addition to the classic French salad celeri-rave remoulade made with raw, shredded celery root tossed with a mustard-mayonnaise dressing, celeriac is excellent in soups, stews and purees. Substitute or combine celery root in any recipe that calls for parsnips, turnips or Jerusalam artichokes. It's delicious mashed with potato (add cubes of celeriac 10 to 15 minutes before the spuds are ready). Julienne the two for a different twist on the potato pancake. Tender slices of celeriac may also be baked as a gratin, braised with meats and sauteed. For a special treat, try them sliced and deep-fried.