Preheat oven to 425°F. In a 10-inch enamel coated cast iron skillet, combine the tomatoes, onion, oil and garlic. Season with 1/4 tsp. salt and pepper. bake for 15 minutes.
In a large bowl, combine the flour, baking powder, soda, 1/4 tsp. salt, and cheese and butter mixture. Add the milk and mix until the batter just comes together. If it's too dry, add more liquid 1 Tbs. at a time.
Drop batter in mounds on top of the tomatoes. Bake 20 minutes more or until the tomatoes bubble and the topping is golden brown.
Grate cheese and butter. Store together in the fridge.