Preheat oven to 400 °F. Spray a 9x13" casserole dish with nonstick spray.
Make corn bread according to package instructions with milk and egg; add 1/2 cup cheddar cheese to the mixture. Spread mixture evenly across the bottom of the casserole dish.
Heat up the package of frozen corn for 5 to 6 minutes; top the cornbread mixture with the corn. Place the dish in the oven and bake for 15 minutes.
While the casserole bakes, shred or chop the brisket and mix with barbecue sauce.
Pull the cornbread from the oven after 15 minutes; poke holes 1 inch apart throughout. Drain the can of ranch-style beans and pour the beans over the top of the cornbread, then top with the brisket and barbecue mix.
Bake an additional 10 to 15 minutes; top with green onions and 1/2 cup cheddar cheese. Serve immediately.