The perfect roast beef recipe, perfect for a Sunday Lunch, let the beef rest for at least half an hour then slice thinly for a tender roast beef dinner. The roast beef sandwich commonly comprises bread, cold roast beef (either the leftovers from a homemade dinner or deli meat), lettuce and mustard, although it would not be uncommon to find horseradish, fresh/powdered chili pepper and even in some cases red onion.