Temperature and timing come first

THE veteran cook's careless phrase ``Oh, I'll just throw in a roast,'' may be both mystifying and intimidating to the kitchen novice. Although roasting is a simple way to cook a large amount of meat, with or without vegetables, correct temperature and timing are of utmost importance. Years ago, American cooks would test oven heat by holding their hand in the oven. Roasting was largely a hit or miss affair. But since the development of good ranges and accurate oven thermometers, the subject of meat temperature has become essential.

Let's start at the beginning. Although you may be tempted to buy economy cuts, the more expensive cut will assure you a tender, juicier result. And always use a meat thermometer. It's difficult to guess at what you can't see. Oven thermometers are seldom reliable and, like the weather, can be capricious.

There are two basic schools of ``how to's'' for roasting meat, depending upon the desired end result. One is slow roasting, geared for the family that prefers its meat medium to well-done. This entails cooking the meat for the indicated time at a constant low heat of 300 to 325 degrees F. The other method involves starting the process at 450 degrees F. (searing in the juices) for 15 minutes, then lowering the heat to 300 degrees F. and continuing to cook until the meat thermometer registers the doneness desired. Both of these methods require a preheated oven.

First, make sure the meat is dry. If you have marinated a tenderloin, for example, drain it well and pat it dry. You may season the outside by shaking the roast in a plastic bag in which you've placed several tablespoons of flour and seasonings of your choice.

Do not use salt, as it draws out the juices. Place a meat thermometer in the thickest part of the roast, avoiding fatty areas and taking care not to touch bone. Put meat or fowl in preheated oven and check the thermometer after half an hour. Set a timer so you won't forget to check. This will give you a feel for how quickly your treasure is progressing.

For maximum, even browning and cooking, place the roast on a cradling-type rack set in a large pan. This allows the heat to circulate evenly, and the drippings are handily caught for gravy.

Always let a roast sit for about 15 minutes after cooking. This allows the juices to disperse in the meat, and also firms the texture, making carving easier.

Some people who like rare meat like it very rare, and a thermometer's charts will not please these diners.

Rare roast beef, if you go by the average thermometer, has an internal temperature of 140 degrees F. Rare beef enthusiasts would consider that medium rare, or even medium. Nearer the mark is 120 degrees F.

Readings for lamb on the charts are high, too, for those who like very pink lamb. According to the average thermometer, lamb should be cooked to 170 or 175 degrees F. That's well done, indeed, and a disappointment to those who like it juicy and pink at 130 to 135 degrees F. A few thermometers begin as low as 130 degrees F., but many start at 140 degrees F.

The following recipe for Roast Leg of Lamb is from ``The Cooking of South-West France,'' by Paula Wolfert (Doubleday, $24.95). Roast Leg of Lamb in the Style of Bordeaux (Gigot `a la Bordelaise) 1 leg of lamb (51/2 pounds) 2 large cloves of garlic, peeled and cut into thin slivers 1 teaspoon coarse (kosher) salt 1/2 teaspoon freshly ground pepper 2 tablespoons rendered goose or duck fat 2 tablespoons grape-seed or French peanut oil 3/4 cup red wine vinegar 2 tablespoons finely chopped shallots 3/4 cup unsalted chicken stock, degreased

Trim off excess fat and tough outer skin from lamb, leaving a thin layer of fat. Make about 10 incisions near leg bone and insert garlic slivers. Rub meat with salt and pepper, then coat with fat and oil. Massage into the meat. Refrigerate, loosely covered with plastic wrap. Remove from refrigerator 2 to 3 hours before roasting. Preheat oven to 500 degrees F. 1-3/4 hours before serving.

Place lamb on grid or rack in large open roasting pan. Set on upper-middle oven shelf. Roast, fat side up, 25 minutes. Remove from oven. Let lamb relax 30 minutes. Meanwhile, in a small, noncorrodible saucepan, combine vinegar and shallots; bring to a boil. Simmer slowly about 20 minutes, or until reduced to 1/3 cup. Strain, reserving shallots (can be done in advance).

Degrease roasting pan. Add vinegar solution and 1/2 cup water to pan juices. Lower oven temperature to 350 degrees F. Return meat to oven and roast 30 minutes longer, basting with vinegar-flavored pan juices every 5 minutes. (Total roasting time should be about 10 minutes to the pound for medium-rare, or 135 to 140 degrees F. on meat thermometer.)

Remove lamb from oven. Add stock and reserved shallots to pan drippings (usually there is not much more than a glaze) and deglaze the pan. Boil up and season lightly. Let lamb rest 5 to 10 minutes. Slice and serve with shallot sauce. Serves 6.

This recipe is from ``Chef Paul Prudhomme's Louisiana Kitchen'' (William Morrow, $19.95). Louisiana Roast Beef 1/4 cup very finely chopped onions 1/4 cup very finely chopped celery 1/4 cup very finely chopped green bell peppers 2 tablespoons unsalted butter or margarine, melted, or vegetable oil 1 teaspoon salt 1 teaspoon white pepper 3/4 teaspoon black pepper 3/4 teaspoon minced garlic 1/2 teaspoon dry mustard 1/2 teaspoon ground red pepper (preferably cayenne) 1 (31/2 to 4-pound) boneless sirloin, top round, or any good-quality beef roast with a layer of fat on top.

In small bowl mix onions, celery, bell peppers, butter, and seasonings.

Place roast in a large pan, fat side up. Make 6 to 12 deep slits in meat (to form pockets) down to a depth of about 1/2 inch from bottom. Do not cut all the way through.

Fill the pockets with vegetable mixture, reserving about 1 tablespoon to rub over top of roast.

Bake roast uncovered at 300 degrees F. about 3 hours, until it reads 160 degrees F., for medium doneness. (For rarer roast, cook until thermometer reads 140 degrees F.) Serve immediately. Serves 6 to 8.