Preheat oven to 425°. Pour oil into a 10-inch cast-iron skillet. Place pan in oven until oil is very hot, about 10 minutes.
In a large bowl, whisk together cornmeal, flour, baking powder, and salt. In a medium bowl, whisk together buttermilk, melted butter, and eggs. Make a well in center of dry ingredients. Add buttermilk mixture; stir until combined. Fold in ½ cup cheese and ⅓ cup corn. Carefully pour batter over hot oil in skillet.
Bake for 10 minutes. Remove from oven; top with tomato, jalapeño, onion, remaining ½ cup cheese, and remaining ⅓ cup corn. Bake until golden brown and a wooden pick inserted in center comes out clean, about 27 minutes more.