In a saucepan, cook sausage and onion until brown and tender; drain off excess fat. Stir in undrained tomatoes, tomato sauce, mushrooms, oregano, garlic powder, 1 teaspoon salt and dash pepper. Simmer, covered, for 5 minutes
Meanwhile, stir together flour, cornmeal, Parmesan cheese, sugar, baking powder and 3/4 teaspoon salt. Make well in center of dry ingredients; add milk, oil and egg, stirring until combined.
Spread batter in bottom of an ungreased 9 x 13-inch casserole dish. Spoon tomato mixture on top. Bake in 400 degree oven for 20 minutes. Sprinkle Cheddar cheese and parsley on top. Bake 5 minutes more. Serves 8.