Fennel, a tall-growing plant with feathery, bright green foliage and yellow flowers, is both an herb and a vegetable. It chiefly is valued for its slightly curved, fragrant grayish-green seeds but also is an interesting vegetable when poached or boiled. It has a warm, sweet taste, similar to anise.

Fennel seeds are used to flavor sausages, soups, breads, pastries, sweet pickles and fish. The delicate leaves are use mostly as a garnish, for enhancing sauces.

Florence fennel, native to Italy, where it is called finocchio, grows to only a foot tall with stalks that resemble celery and a bulbous white base. The stalks can be sliced and served with other raw vegetables and an herb dip. They also may be braised or baked, in any way suitable for celery.

Here are some recipes using fennel, both as an herb and a vegetable. The first is a salad of French origin, and combines the licorice taste of raw fennel with the crunch of chopped walnuts. FENNEL-WALNUT SALAD (6 servings) 1 1/2 pounds small fennel bulbs Ice water 1/4 cup whipping cream 2 tablespoons fresh lemon juice 1 tablespoon Dijon mustard Salt and freshly ground white pepper 3/4 cup coarsely chopped walnuts 1/2 tablespoon fresh or 1/2 teaspoon dried chervil or tarragon

Chill a medium size salad bowl. Remove fennel tops. Snip and set aside 1 tablespoon for garnish. Cut away hard outer stalks. Trim base. Slice blub crosswise into very thin slices. Chill in bowl of ice water for 5 minutes. Drain well and dry on paper towels.

Combine cream, lemon juice and mustard. Season with salt and pepper to taste. Just before serving, combine fennel and walnuts in chilled bowl. Add dressing and toss gently. Garnish with herbs and fennel leaves. FENNEL MEATBALLS IN SAUCE (6 servings) Meatballs: 2/3 cup fine soft bread crumbs 1/4 cup beef broth or water 1 1/2 pounds lean ground beef 1/4 cup chopped parsley 1 egg, slightly beaten 1 large clove garlic, crushed 1 1/2 teaspoons fennel seed, crushed 1 teaspoon salt 3/4 teaspoon oregano 1/4 teaspoon fresh ground pepper Tomato Sauce: 2 cups onions, minced 1 large green pepper, cut in 1/2-inch strips lengthwise 1/2 cup chopped parsley 1 can (14 or 16 ounces) tomatoes 1 can (15 ounces) tomato sauce 2 large cloves garlic, crushed 1 teaspoon mixed Italian herbs 1/2 teaspoon sugar 1/4 teaspoon fresh ground pepper 1/4 teaspoon oregano

In large bowl soak bread crumbs in broth or water. Add remaining meatball ingredients and mix thoroughly. Shape in 2-inch balls. In large heavy kettle over medium heat, brown meatballs in their own fat. Do not crowd. Remove from kettle. Discard all but 3 tablespoons drippings. d

To make sauce, add onions to kettle and saute, stirring occasionally, until tender. Stir in remaining ingredients. Add meatballs. Partly cover and simmer, stirring occasionally until sauce is thick, about 1 1/2 hours. Serve over cooked pasta. ROAST PORK WITH FENNEL SEEDS (6 to 10 servings) 4-pound, center cut pork loin roast 3 cloves garlic, slivered Salt and freshly ground pepper 1 tablespoon fennel seeds, crushed 2 small white onions, peeled 1/2 cup chicken broth

Have butcher cut away chine bone to make roast easier to carve. Preheat oven to 400 degrees. Make gashes at various points in fat of roast and between meat and rib bones. Stud with garlic slivers. Sprinkle roast with salt and pepper to taste. Rub fennel seeds on top of roast.

Place roast meaty side down in shallow baking pan. Chop chine bone in pieces and scatter around roast. Add onions.

Bake, basting frequently for 30 minutes. Turn bone side down. Continue baking and basting 30 minutes more. Pour off all fat from pan. Add chicken broth and cover roast lightly with foil. Bake 30 to 45 minutes longer. Serve hot or cold.