Melt butter in a medium saucepan over medium-high heat. Add onion, and sauté 5 minutes or until tender. (Do not brown onion.) Remove pan from heat. Stir in cornbread mix, 1/2 cup cheese, next 4 ingredients, and, if desired, dill.
Coat an 8-inch square pan with cooking spray; pour mixture into pan.
Bake at 450° for 25 minutes. Sprinkle evenly with remaining 1/2 cup cheese, and bake 5 more minutes or until a wooden pick inserted into center comes out clean. Cool slightly before cutting into squares.