Cook the spaghetti: Boil salted water and cook spaghetti until al dente. Drain and set aside.
Mix the creamy layer: Combine softened cream cheese and sour cream in a bowl. Stir until smooth.
Cook the beef and sauce: Brown the ground beef in a skillet over medium heat (7–10 minutes). Drain excess fat, then stir in marinara sauce, garlic powder, and Italian seasoning. Simmer for 5 minutes. Season with salt and pepper.
Assemble the casserole: In a greased 9x13-inch baking dish, layer half the spaghetti, spread the cream cheese mixture over it, then add half the beef sauce and 1 cup of mozzarella.
Repeat the layers: Add the remaining spaghetti, beef sauce, and the final cup of mozzarella.
Bake uncovered at 350°F (175°C) for 25–30 minutes until golden and bubbly. Let rest for 5 minutes before serving.