In a skillet, cook the meat, mushroom, green pepper, onion and garlic until the meat is browned and the vegetables are tender; drain. Stir in tomato sauce, fennel, salt and oregano. Simmer for 1-2 minutes.
Meanwhile, warm pitas in microwave. Top each with meat mixture; sprinkle with cheese. Microwave or broil until cheese is melted. Cut into quarters.