1Preheat oven to 350 degrees F (175 degrees C). Grease a deep 9x13 inch baking dish.
2Bring a large pot of salted water to a boil over high heat. Stir in the spaghetti. Boil the pasta until cooked through but still firm to the bite, 8 to 10 minutes. Drain well.
3Brown ground beef and onion in a large skillet over high heat; drain fat. Stir in the spaghetti sauce, sour cream, and 1 cup of the mozzarella. Mix in the cooked pasta. Transfer pasta mixture to prepared baking dish. Top with remaining 1 cup of mozzarella and the Parmesan cheese. Cover pan with aluminum foil.
4Bake in preheated oven until hot and bubbly, about 30 minutes.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.