Beef Enchilada Casserole

Name: Beef Enchilada Casserole
Ingredients: 1 tbsp. extra-virgin olive oil; 1 1/2 lb. (90% lean) ground beef; 1 small red onion, chopped, plus more for garnish; 1 medium green bell pepper, cored and finely chopped; 1 tsp. (or more) kosher salt; 1 tbsp. chili powder; 1 tsp. garlic powder; 1 (15-oz.) can black beans, rinsed and drained; 2 c. frozen corn (from a 12-oz. bag); 2 (10-oz.) cans red enchilada sauce, divided; 2 1/2 c. shredded cheddar; 2 1/2 c. shredded Monterey Jack cheese; 18 corn tortillas; Finely chopped fresh cilantro, chopped avocado, and sour cream, for serving