Preheat and Prepare: If using the oven, preheat it to 350°F. Cook the rotini pasta according to the package instructions until it is firm al dente, as it will continue to cook in the casserole.
Cook the Meat: In a large skillet, brown the ground beef and Italian sausage over medium heat. Once browned, add the chopped onions and mushrooms. Continue to cook until the vegetables are softened. Stir in the minced garlic and sauté for an additional 2-3 minutes.
Prepare the Sauce: Mix in the Italian seasoning, diced tomatoes, and tomato sauce. Allow the mixture to simmer briefly, then set it aside to cool for about 10 minutes.
Assemble the Casserole: Lightly grease a 9×13-inch casserole dish or the inside of the slow cooker. Spread a thin layer of the sauce mixture on the bottom. Add the cooked pasta on top, followed by a layer of the shredded Italian cheese blend. Pour the remaining sauce over the pasta.
Cook the Casserole: Cover and bake in the preheated oven for 45 minutes, or if using a slow cooker, cook on low for 1 1/2 to 2 hours.
Add Final Cheese Layer: Once done, uncover and sprinkle the shredded old Cheddar cheese over the top. Cover again until the cheese is melted and bubbly.
Serve: Dish up the casserole while hot and enjoy!