Here's a nice change from typical Greek gyro sandwiches. It's best to pack the yogurt sauce in a separate container so the sandwich won't become soggy before lunchtime; either drizzle the sauce into the pita halves just before eating, or serve it on the side for dunking.
1/2 cup plain fat-free yogurt
1 tablespoon tahini (sesame-seed paste)
1/2 teaspoon chile garlic sauce (such as Lee Kum Kee)
Combine first 5 ingredients, stirring with a whisk.
Fill each pita half with about 1/3 cup Simple Roasted Leg of Lamb, 1/4 cup onion, and 1/4 cup cucumber. Divide tomato slices evenly among pita halves. Wrap sandwiches in foil or parchment paper; chill. Serve with sauce.