Very quick and easy, and a great way to add variety to leftovers.
1/2 to 3/4 pound leftover cooked roast beef, cut in strips
2 tablespoons fresh lemon juice
2 tablespoons minced onion
1-1/2 cups sour cream
1 garlic clove, minced
2 teaspoons Dijon-style prepared mustard
Salt and pepper to taste
4 to 6 ripe beefsteak tomatoes, sliced
In a large bowl, combine the beef with the lemon juice, onion, sour cream, garlic, mustard, and salt and pepper. Arrange the tomatoes on a large serving platter and spoon the beef mixture on top.