Preheat oven to 350°. Prepare cornbread batter according to package directions. Spread into a greased 11x7-in. baking dish.
In a large skillet, cook beef over medium heat until no longer pink; drain. Add tomatoes, vegetables, tomato paste and seasoning. Bring to a boil. Reduce heat; simmer, uncovered, for 5 minutes. Pour over batter. Sprinkle with cheese.
Bake, uncovered, until golden brown, 25-30 minutes. Sprinkle with onions.
Freeze option: Omit onion topping. Cool baked casserole; wrap and freeze. To use, partially thaw in refrigerator overnight. Remove 30 minutes before baking. Preheat oven to 350°; bake as directed, increasing time as necessary for a thermometer inserted in center to read 165°. Sprinkle cooked casserole with onions.