01 - Set oven temperature to 350°F (180°C) and allow to preheat fully.
02 - In a large skillet over medium heat, cook the ground beef or turkey until thoroughly browned. Drain excess fat from the pan.
03 - Add diced onion, minced garlic, and red bell pepper to the browned meat. Sauté for 3 to 4 minutes until vegetables are softened.
04 - Add frozen mixed vegetables, drained diced tomatoes, oregano, basil, salt, and black pepper. Stir thoroughly and simmer the mixture for 5 minutes.
05 - Remove skillet from heat. Fold in the cooked pasta and sour cream, mixing until distributed evenly.
06 - Fold in half of the cheddar and mozzarella cheeses until just combined.
07 - Grease a 9x13-inch baking dish, then spoon the prepared mixture into the dish, spreading evenly.
08 - Top casserole evenly with the remaining cheddar and mozzarella cheeses. If using, mix breadcrumbs with melted butter and sprinkle over the cheese layer.
09 - Place baking dish in the oven and bake for 35 to 40 minutes, until cheeses are bubbling and the top is golden brown.
10 - Remove from oven and let rest at room temperature for 5 to 10 minutes prior to serving.