Preheat oven to 400 degrees F. Grease a 9x13-inch baking pan with nonstick spray; set aside.
In a large skillet, cook the beef and onions until the meat is browned and the onions are soft and translucent. Mix in the Mrs. Dash Chili Seasoning Mix, tomato sauce, diced tomatoes and beans. Allow to come to a boil, then reduce the heat and simmer for 10 minutes, stirring occasionally.
Meanwhile, prepare the cornbread. In a mixing bowl, stir together the cornbread mix, milk and eggs until just combined. Mixture will be lumpy. Let set to rise for 3-4 minutes. Spread in the prepared baking pan and bake for 12-15 minutes. Remove from oven and let set for 5 minutes.
Evenly spread about ¾ of the shredded cheese over the cornbread. Layer the meat mixture over top, then sprinkle with the rest of the cheese. Return to the oven for about 5-8 minutes, until heated through and cheese is melted.
Remove from the oven and let set for 3 minutes before slicing into squares and serving. Serve with corn chips, green onions, sour cream and additional shredded cheese.