Cook beef and onion: Preheat oven to 450°F. Heat 2 tablespoons of the oil in a 12-inch cast-iron skillet over medium-high. Add beef, onion, salt, and pepper; cook, stirring occasionally and crumbling beef with a wooden spoon, until beef is browned and onions have softened, about 6 minutes.
Add seasonings: Add tomato paste, garlic, chili powder, and cumin; cook, stirring often, until fragrant and fully incorporated, about 1 minute.
Add stock, beans, and tomatoes: Stir in chicken stock, black beans, and tomatoes until well combined.
Cook chili mixture, and add corn: Bring to a boil over medium-high; cook, stirring occasionally, until liquid just slightly thickens and reduces, about 5 minutes. Stir in frozen corn, and remove from heat.
Make cornbread mixture: Whisk together cornmeal mix, milk, egg, and remaining ¼ cup oil until smooth and combined.
Sprinkle cheese on chili: Sprinkle pepper Jack evenly over beef mixture in skillet.
Add cornbread mixture to casserole: Pour cornbread batter evenly over pepper Jack.
Bake: Bake in preheated oven until cornbread is golden brown, 20 to 25 minutes. Serve immediately.
