Preheat the oven. Set to 375°F (190°C). Grease a 9x13-inch baking dish.
Cook the spaghetti. Boil in salted water until just shy of al dente (about 1–2 minutes less than package).
Drain and toss with a little olive oil so it doesn’t clump.
Brown the meat. In a large skillet, heat olive oil over medium-high. Add onion and cook 3 minutes. Add garlic and cook 30 seconds.
Add ground beef, season with salt and pepper, and cook until browned. Drain excess fat if needed.
Build the creamy meat sauce. Stir in marinara, Italian seasoning, and crushed red pepper. Simmer 3–5 minutes.
Reduce heat to low and fold in 1/2 cup Alfredo to make it silky.
Mix the Alfredo layer. In a bowl, combine remaining Alfredo sauce with ricotta (if using) and 1/4 cup Parmesan. Season with a pinch of salt and pepper.
Layer the bake. Add half the spaghetti to the baking dish. Spoon half the Alfredo mixture over the noodles and gently toss to coat.
Top with half the meat sauce and a sprinkle of mozzarella. Repeat with remaining spaghetti, Alfredo mixture, meat sauce, and mozzarella. Finish with remaining Parmesan.
Bake. Cover loosely with foil and bake 15 minutes.
Remove foil and bake another 10–15 minutes, until the top is golden and bubbly.
Rest and serve. Let it sit 5–10 minutes so it sets. Garnish with parsley and serve warm.