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