Combine the baking mix and 1/2 cup of cheese in a large bowl; set aside. In a small bowl, beat egg and milk; set aside. In a small skillet, saute onion in butter until tender; add to egg mixture. Stir into cheese mixture just until moistened. Fill greased or paper-lined muffin cups three-fourths full. Top with the sesame seeds and remaining cheese. Bake at 400° for 18-20 minutes or until muffins test done. Cool in pan 10 minutes. Serve warm.