Preheat your oven to 350°F (175°C).

Cook the spaghetti according to package instructions until al dente, then drain and set aside.

In a large skillet, brown the ground beef over medium heat until fully cooked, breaking it apart with a spoon.

Add the diced onion and minced garlic to the skillet and sauté for 3-4 minutes until softened.

In a medium bowl, mix the softened cream cheese, marinara sauce, milk, Italian seasoning, salt, and pepper until smooth.

Add the cooked spaghetti and the beef mixture into a large mixing bowl and pour the cream cheese sauce over them.

Stir gently to combine all ingredients evenly.

Transfer the mixture into a greased 9x13 inch baking dish and sprinkle shredded mozzarella cheese on top.

Bake the casserole in the preheated oven for 25-30 minutes until bubbly and the cheese is melted and slightly golden.

Remove from oven and let it cool for 5 minutes before serving.

Sprinkle grated Parmesan cheese and chopped fresh parsley over the casserole for garnish.