For yogurt sauce, in a small bowl stir together the yogurt and tahini; set aside.Advertisement
In a small bowl stir together the salt, pepper, allspice, cardamom, and nutmeg. In a large skillet cook onion and garlic in hot oil for 3 to 4 minutes or until onion is just tender. Add spice mixture to skillet. Cook and stir for 1 minute more. Add meat to skillet; cook and stir until meat is heated through. Stir in the lemon juice.
Spoon meat mixture evenly into pita halves. Add tomato slices and, if desired, avocado slices. Serve with yogurt sauce.