Cook spaghetti according to package directions until al dente; drain and set aside.

Preheat oven to 350°F (175°C).

In a large skillet or Dutch oven, heat olive oil over medium-high heat. Add ground beef, breaking it apart, and cook until browned; drain excess fat.

Stir in diced onion and cook until softened, about 5 minutes, then add minced garlic and cook for another minute until fragrant.

Mix in crushed tomatoes, tomato sauce, tomato paste, Italian seasoning, salt, and pepper; bring to a simmer, then reduce heat and let it cook for at least 15 minutes, or up to 30 minutes, to allow flavors to meld.

In a mixing bowl, combine ricotta cheese, softened cream cheese, sour cream, grated Parmesan cheese, egg, and milk until smooth.

Spread half of the cooked spaghetti in the bottom of a greased 9x13 inch baking dish.

Evenly spread the entire ricotta cheese mixture over the spaghetti layer.

Top with the remaining spaghetti, then pour the meat sauce over it, spreading evenly.

Sprinkle 1.5 cups of shredded mozzarella cheese over the meat sauce.

Bake for 25 minutes, then remove from oven, sprinkle with the remaining 0.5 cup mozzarella cheese, and bake for another 10 minutes, or until cheese is melted and bubbly.

Let rest for 5-10 minutes before serving.