Preheat oven to 400°F. Grease square baking dish, 8x8x2-inches. Cook and stir ground beef and onion in 10" skillet until beef is brown; drain. Stir in seasoning mix; spoon into baking dish.
Beat baking mix, milk, and eggs with wire whisk or hand beater until almost smooth, about 1 minute. Pour over beef in baking dish.
Bake until knife inserted in center comes out clean, about 25 minutes. Sprinkle with cheese. Bake until cheese is melted, about 2 minutes. Cool 5 minutes. Garnish with lettuce, tomato, olives and, if desired, sour cream.