The thick outer layer of fat on meat is not removed completely before cooking; it is trimmed thinner. This fat melts during cooking and runs down over the meat, moistening the surface; a piece with all the fat removed dries out much faster, especially in the oven or on the grill. The proper approach is to trim the cap down to about half a centimeter with a knife and score it crosswise. Any remaining fat can be cut off before serving if desired. The choice depends on the cut and cooking time.
The fat cap has two jobs. The first is protection: in the oven or over coals, the meat receives heat from above through this layer first, so the muscle fibers underneath do not burn directly. The second is moistening; as the fat melts and runs down, it coats the surface and slows water evaporation. If the cap is removed completely, these two forms of protection are lost. That is why the fat cap is kept on top throughout cooking, especially on long-cooking cuts such as leg of lamb, beef roast, and ribs. For the same reason, these cuts are placed in the oven with the fat cap facing up; on a piece placed upside down, the melted fat runs into the pan without touching the top of the meat.
The rough measurement is half a centimeter. A thicker layer will not have time to melt and will remain as a soft, greasy outer layer; a thinner layer will be used up before cooking is finished, and protection will end too early. After trimming the cap thinner, score it crosswise with a knife, but do not cut down to the meat—cut only the fat. Scoring has two purposes: it helps the fat melt more easily and prevents the fat, as it contracts during cooking, from tightening and curling the meat. A spacing of one and a half to two centimeters between cuts is enough.
The same choice does not apply to every cut. On thin slices cooked quickly, such as schnitzel and thin-cut steak, the fat does not have time to melt, so most of it is removed. Fat intended for ground meat is calculated separately and counted as part of the ratio. For dishes cooked in liquid, such as boiled dishes and stews, the cap is removed at the start because fat that does not melt into the water collects on the surface and makes the dish heavy; if it was not removed, the fat that collects on the surface is skimmed off with a spoon at the end of cooking.
Grilling has a separate consideration. When melted fat drips onto the coals, the flames flare up and leave soot on the surface of the meat; the soot replaces the roasted flavor with a bitter taste. That is why cuts with a fat cap are not placed directly over the coals; they are moved a little to the side, and when the flames flare up, the meat is temporarily moved away. Sprinkling water on the coals is not a solution; it only raises ash. If the fat cap has been trimmed thinner beforehand, this problem is reduced from the start.
After cooking, the cap can be cut off if desired; its job is done at that point. On leg of lamb and roasts, the remaining fat is usually served with the slices because it adds flavor to the meat. The trimmed fat is not discarded either: chop it into small pieces and render it over low heat to use in other dishes. Take care with storage: fat turns rancid faster than muscle tissue, so a thick piece with its fat cap should be cooked within three days when refrigerated. If it will be frozen, leave the cap untrimmed, because the layer of fat protects the meat underneath to some extent from freezer burn.