Preheat oven to 375℉ and prepare a 9x13 baking dish by spraying it with a non-stick cooking spray. Then set aside.
In a large pot, cook pasta just before it is al dente according to the directions on the package. Then drain and set aside.
Heat olive oil in a large skillet over medium-high heat. Add onions and carrots, cook 2-3 minutes. Add sausage and break up with a spatula. Cook until browned and no longer pink. Drain grease, then remove sausage and onions.
Lower heat to medium and melt butter. Add garlic and sauté for 30 seconds, slowly start to whisk in heavy cream and black pepper. Whisk continuously until the sauce is well combined.
When the sauce starts to bubble, slowly whisk in the parmesan cheese. Whisk continuously until the sauce is smooth.
Add marinara sauce, diced tomatoes, cooked sausage, and spinach to alfredo sauce. Mix until well combines. Cook until spinach wilts, stirring occasionally.
Add the sauce to the pasta and toss to coat. Then, add half of the sauce and pasta mixture to the prepared baking dish. Top with 1 cup of mozzarella cheese. Add the remaining pasta and sauce, then top with remaining cheese.
Cover with aluminum foil and bake for 25 minutes. Remove the foil and continue to bake for 15 minutes.
Let rest for 10 minutes before serving and enjoy!