Reheating any meat makes it tough, dries it out, and causes your guests to run screaming from your house*. Must you follow through with your evil plan?

If you were doing a braise or a lamb stew, then yes, cooking it a day ahead and reheating the next day might actually produce better flavor. But any method of dry cooking – roasting, grilling, or broiling – is going to produce a leg or roast that is perfect shortly after it comes from the oven, but that doesn't want to be reheated. What meat benefits from being cooked twice?

Also, we assume (for you as for everyone) that you want your leg of lamb roasted to very rare (110°F to 115°F when you take it from the oven – a state only acceptable for beef and lamb), rare (115°F to 120 °F), or medium rare 125°F to 130 °F)**. Unless you reheat it at least to those lower temperatures, it is going to taste tepid in the mouth (keep reading to know why you really don't want to do that), so you will essentially be cooking it fully, twice. If you intended to serve well-done lamb, then your plan is fine, brilliant even.

You would not serve it cold, in any event, as lamb fat is gross when cool. In The Complete Meat Cookbook, Bruce Aidells and Denis Kelly say that many chefs use warm platters and plates to ensure that lamb gets to diners while still hot. They even caution against serving ice water with lamb, as it can cause the fat to congeal in your mouth, "with unpleasant results."

You can certainly use leftover lamb in any of thousands of recipes for stew, curry, tagines, tacos, or anything with a sauce or gravy. But to roast a wonderful piece of meat and intentionally use it as one big leftover? Here we must put out little foot down. This is one case where you need to plan the timing of the meal around your main course or serve something else. Sorry.

*Well, it makes them want to run screaming from your house. They are probably too polite to actually do so.

**For medium, keep cooking until 130°F to 140 °F; for medium-well, take the lamb out of the oven between 145°F and 155°F; and for well-done, play a round of golf, sip some lemonade, polish your shoes, maybe take in a basketball game, and if your tummy starts to rumble by this point, think about checking on that roast.