Preheat the oven to 400°F. Slice the bell peppers in half from top to bottom, straight through the stem, and scoop out the seeds and white ribs so the halves look like little canoes.
Place the empty pepper halves cut-side down in a 9x13-inch baking dish. Roast for 10 minutes — this pre-roast is the secret to keeping the peppers crunchy instead of soggy.
Meanwhile, brown the ground beef in a large skillet over medium-high heat, breaking it up with a spatula, until fully browned, 5 to 7 minutes. Drain excess fat but leave a little for flavor.
Push the beef to one side and cook the diced onion for 3 to 4 minutes until soft and translucent. Add the garlic and cook for 30 seconds until fragrant, then stir everything together.
Stir in the taco seasoning, black beans, corn, and salsa. Let it bubble gently for 2 to 3 minutes, then taste and add salt and pepper only if needed.
Flip the roasted peppers cut-side up. Spoon the beef mixture generously into each half, mounding it just above the rim.
Sprinkle 2 to 3 tablespoons of shredded cheese over each pepper, covering the filling completely.
Bake for 20 to 25 minutes until the cheese is melted, bubbly, and starting to brown at the edges. If you want an extra-golden top, broil for the last 1 to 2 minutes.
Let the peppers rest for 5 minutes before serving. Serve with sour cream, cilantro, avocado, salsa, jalapeños, or hot sauce.