01 - Set oven to 350°F (180°C) and allow to fully preheat.
02 - In a large skillet over medium heat, cook ground beef or turkey until thoroughly browned. Drain off excess fat.
03 - Add diced onion, minced garlic, and bell pepper to the skillet. Cook for 3 to 4 minutes, stirring occasionally, until vegetables are softened.
04 - Add frozen mixed vegetables, canned diced tomatoes, dried oregano, dried basil, salt, and black pepper. Stir well and simmer mixture for 5 minutes.
05 - Remove skillet from heat. Stir in cooked pasta and sour cream until everything is evenly incorporated.
06 - Fold in half of the shredded cheddar and mozzarella cheese to the mixture.
07 - Transfer the mixture into a greased 9x13-inch baking dish, spreading it out evenly.
08 - Sprinkle remaining cheddar and mozzarella cheese over the casserole. Mix breadcrumbs with melted butter, if using, then sprinkle over the top.
09 - Place baking dish in the oven and bake for 35 to 40 minutes, until the cheese is bubbly and the top is golden brown.
10 - Allow casserole to cool for 5 to 10 minutes before serving for easier slicing.