01 - Preheat oven to 350°F (180°C).
02 - Heat a large skillet over medium heat. Add the ground beef or turkey and cook, breaking it apart with a spoon, until well browned. Drain excess fat.
03 - Add diced onion, minced garlic, and diced red bell pepper to the skillet. Sauté for 3 to 4 minutes until vegetables are softened.
04 - Incorporate frozen mixed vegetables, drained diced tomatoes, dried oregano, basil, salt, and black pepper. Stir well and allow the mixture to simmer for 5 minutes.
05 - Remove the skillet from heat. Add the cooked pasta and sour cream, stirring until evenly blended.
06 - Gently fold in half of the shredded cheddar and mozzarella cheeses.
07 - Transfer the prepared mixture into a greased 9x13-inch (23x33 cm) baking dish, spreading it evenly.
08 - Scatter the remaining cheddar and mozzarella cheese over the casserole. If using, mix breadcrumbs with melted butter and sprinkle over the top.
09 - Place the dish in the oven and bake for 35 to 40 minutes, until the cheese is bubbling and the topping is golden brown.
10 - Allow the casserole to cool for 5 to 10 minutes before slicing and serving.