In large cast iron skillet, sauté onions in olive oil over med/low heat. Add in garlic and continue sautéing until onions are tender.
Add cooked meatballs.
Add marinara sauce and tsp Italian seasoning.
Heat over med/low heat for about 20-30 minutes.
In separate large pot, cook spaghetti noodles according to package directions (under cook by 2 minutes).
With large slotted spoon, scoop meatballs from sauce and set aside.
Scoop cooked spaghetti (with large slotted spoon) into marinara sauce. Add in about 1/2 cup pasta cooking water. Gently stir until pasta is fully coated with sauce.
Add cooked meatballs to top of spaghetti.
Top with grated cheese & parmesan cheese.
Bake at 350* for approx 20-30 minutes or until hot and bubbly and cheese is melted.