Filling: Preheat oven to 400 degrees. Heat oil in a large ovenproof saute pan over medium-high heat. Add onion, carrots, and garlic, and season with salt. Cook, stirring occasionally, until onion is translucent, about 5 minutes. Add chili powder and tomato paste; cook, stirring, 30 seconds.

Stir in beef and cook, breaking up with the back of a spoon, until no longer red, about 5 minutes. Add tomatoes with their juices and beans, bring to a boil, and cook 2 minutes. Remove from heat.

Topping: Whisk together flour, cornmeal, sugar, salt, and baking powder in a large bowl. Whisk in buttermilk, eggs, and butter. Spoon mixture over top of filling, spreading almost to edges of pan.

Transfer to oven and bake until a tester inserted in topping comes out clean, about 20 minutes. Let rest 20 minutes before serving.