1. Heat oven to 400°F. Combine cornmeal, flour, baking soda, baking powder and salt in mixing bowl. Combine buttermilk and 2 Tbsp oil in large measuring cup or small bowl. Add eggs and beat until just mixed. Pour over dry ingredients and mix well. Add cheese, corn, onion and sage and mix very thoroughly.
2. Place remaining oil in a 9-inch square baking dish. Place in oven and heat for 2 minutes, remove the baking dish from oven and swirl oil around to coat interior of the pan. Scrape batter evenly into pan.
3. Bake 40 minutes, until crusty and browned. Let stand 5 minutes before slicing into squares with sharp knife and lift them out with offset spatula; the center may be crumbly. Serve hot with butter if you like.