Preheat the oven to 375°F (190°C).
Clean the portobello caps by removing stems and scraping out the gills; place caps on a baking sheet, brush both sides with 1 tablespoon olive oil, and season with salt and pepper. Set aside.
In a skillet over medium heat, brown the ground beef until fully cooked; transfer to a bowl and drain excess fat if needed.
Add the remaining 1 tablespoon olive oil to the skillet. When hot, add the diced onion and red bell pepper, season with salt and pepper, and sauté until soft and lightly golden, about 5–7 minutes.
Add the minced garlic and taco seasoning to the pan and cook for about 30 seconds until fragrant.
Return the cooked ground beef to the skillet and stir in the tomato sauce, frozen corn, and cooked black beans; cook for 2–3 minutes until heated through. Taste and adjust salt and pepper if needed.
Spoon the taco meat mixture evenly into each prepared portobello cap, then top each with the grated cheddar cheese.
Bake in the preheated oven for 15 minutes, or until mushrooms are tender and cheese is melted. If desired, broil 1–2 minutes to brown the cheese—watch carefully.
Remove from oven and garnish with fresh herbs if using; serve warm.
