01 - Preheat oven to 350°F (180°C).
02 - In a large skillet over medium heat, cook ground beef or turkey until browned. Drain excess fat.
03 - Add diced onion, minced garlic, and red bell pepper to the skillet. Cook for 3 to 4 minutes until softened.
04 - Add frozen mixed vegetables, drained diced tomatoes, oregano, basil, salt, and pepper. Stir well and simmer for 5 minutes.
05 - Remove skillet from heat. Stir in cooked pasta and sour cream until the mixture is evenly blended.
06 - Fold in half of the shredded cheddar and mozzarella cheese into the mixture.
07 - Transfer the mixture to a greased 9x13-inch baking dish, spreading it evenly.
08 - Sprinkle the remaining cheddar and mozzarella cheese over the top. For added crunch, mix breadcrumbs with melted butter if using and scatter over the cheese.
09 - Bake for 35 to 40 minutes, or until the cheese is bubbling and the topping is golden brown.
10 - Allow casserole to cool for 5 to 10 minutes before serving.