Preheat oven to 400°. Heat oil a 10-in. cast-iron or other ovenproof skillet over medium-high heat. Add onion and pepper; cook and stir until tender, 2-3 minutes. Add garlic; cook 1 minute longer. Stir in beans, enchilada sauce, corn, chili powder, cumin and pepper. Stir in tortillas.
Bring to a boil. Reduce heat; simmer, uncovered, until tortillas are softened, 3-5 minutes. Sprinkle with cheese. Bake, uncovered, until bubbly and cheese is melted, 3-5 minutes. If desired, garnish with optional ingredients.