Preheat the Oven - Set the oven rack to the center position. Preheat to 375ºF (191ºC).

Prepare the Bell Peppers - Cut the tops off of each bell pepper, leaving about 3 inches of the bottom for stuffing. Remove the membrane and seeds. Chop the top pieces into ¼" dice, about 1 cup. Set aside. Grease the sides and bottom of a 12-inch cast-iron skillet or casserole baking dish with olive oil. Arrange the peppers inside. Lightly season the insides of the peppers with salt and pepper.

Prepare the Aromatics - Chop the onion into ¼" dice. Mince the garlic. Cut the serrano pepper in half, remove the seeds, membrane, and stem, then mince.

Saute the Aromatics - Heat a large skillet over medium-high heat. Add 1 tablespoon of olive oil. Once hot, add diced onion, minced garlic, and minced serrano pepper. Saute until tender, about 3 minutes.

Cook the Beef - Add in the ground beef, breaking it into smaller pieces. Cook until no longer pink, about 4 minutes. To season, add the cumin, salt, pepper, smoked paprika, and chili powder, and stir to combine.

Cook the Vegetables - Add in 1 cup of diced bell peppers, and saute for 1 minute. Add in the black beans, corn, tomatoes, and green onion, stir, and cook for 1 minute. Turn off the heat and add in ¾ cup cheese, stir to combine. Taste the beef filling, and add more salt and pepper as needed.

Stuff the Peppers - Add about ¾ cup of filling, enough to fill each bell pepper.

Bake the Peppers - Bake until the peppers are tender when pierced with a fork, about 30 minutes. Remove them from the oven and sprinkle about 1 teaspoon of cheese on top of each one. Bake until the cheese is melted, about 2 minutes.

Garnish and Serve - Serve garnished with green onions and thinly sliced serrano peppers.