Set the oven rack to the center position. Preheat to 375ºF (191ºC).
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-inch 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.
Chop the onion into a ¼-inch dice. Mince the garlic. Cut the serrano pepper in half, remove the seeds, membrane, and stem, then mince.
Heat a large skillet over medium-high heat. Add in 1 tablespoon olive oil. Once hot add diced onion, minced garlic, and minced serrano pepper. Saute until onions are tender, about 3 minutes.
Add in the ground beef, breaking into smaller pieces. Cook until no longer pink, about 4 minutes. To season the beef, add the cumin, salt, pepper, smoked paprika, and chili powder, stir to combine.
Add in 1 cup of diced bell peppers, 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, add more salt and pepper as needed.
Add about ¾ cup of filling, of enough to fill each bell pepper.
Bake until the peppers are tender when pierced with a fork, about 30 minutes. Remove the peppers from the oven and sprinkle about 1 teaspoon of cheese on top of each one. Bake until the cheese is melted, about 2 minutes.
Serve stuffed peppers garnished with green onions and thinly sliced serrano peppers.
Make it Keto or Low Carb: Omit the corn and beans and substitute with zucchini, spinach, carrots, or kale.