Cook spaghetti: Boil in salted water until al dente. Drain and toss with melted butter.
Make meat sauce: Brown the meat and onion, drain grease, add garlic, sauce, and seasonings. Simmer 5–10 minutes.
Combine noodles and milk: In a large bowl, toss spaghetti with milk (this keeps it moist and helps it set while baking).
Assemble: Spread half the spaghetti in a greased 9x13 pan. Add half the sauce, then half the cheeses. Repeat layers.
Bake at 350°F: Covered: 25–30 minutes. Uncovered: 10–15 minutes more until bubbly and golden.