01 - Preheat oven to 350°F (180°C).
02 - In a large skillet over medium heat, cook ground beef or turkey until browned, breaking it up with a spoon. Drain any excess fat.
03 - Add diced onion, minced garlic, and diced red bell pepper to the skillet. Sauté for 3 to 4 minutes until vegetables have softened.
04 - Incorporate frozen mixed vegetables, drained diced tomatoes, dried oregano, dried basil, salt, and black pepper. Stir well and simmer for 5 minutes.
05 - Remove skillet from heat. Stir in cooked pasta and sour cream until the mixture is thoroughly combined.
06 - Fold in half of the shredded cheddar and mozzarella cheese, mixing until evenly distributed.
07 - Transfer the mixture into a greased 9x13-inch baking dish, spreading it out evenly.
08 - Top with the remaining cheddar and mozzarella cheese. If using, mix breadcrumbs with melted butter and sprinkle over the cheese layer.
09 - Place the dish in the oven and bake for 35 to 40 minutes until the cheese is bubbly and the top is golden brown.
10 - Allow the casserole to cool for 5 to 10 minutes before serving to help set the layers.