Preheat the oven to 350 degrees F (175 degrees C).

Crumble ground beef into a large cast-iron skillet over medium-high heat; cook, stirring frequently, until evenly brown. Drain grease. Add onion to the skillet; cook and stir until onion is translucent. Add bell peppers, beans, and fajita seasoning; cook and stir until heated through. Spread into an even layer in the skillet.

Combine cornbread mix, egg, and milk in a bowl according to package directions; spoon over beef mixture, spreading evenly.

Bake in the preheated oven until a toothpick inserted into cornbread layer comes out clean, about 20 minutes. Cool before serving, 3 to 5 minutes.