In a large skillet, heat olive oil over medium heat. Add diced onion and bell pepper, cooking until softened, about 5 minutes.
Add the ground beef or turkey to the skillet, breaking it up with a spoon. Cook until browned and cooked through, about 7-10 minutes. Drain excess fat if necessary.
Stir in the taco seasoning mix, black beans, corn, and diced tomatoes. Cook for an additional 5 minutes, allowing the flavors to meld.
Add the cooked rice to the skillet, stirring until well combined. Season with salt and pepper to taste.
Sprinkle shredded cheddar cheese over the top, cover the skillet, and let it cook for another 2-3 minutes until the cheese is melted.
Remove from heat and garnish with fresh cilantro if desired. Serve warm with sour cream on the side.