Heat oven to 375°F and lightly grease a 9x13-inch baking dish.
Boil spaghetti in well-salted water for 6 minutes, just shy of al dente; drain.
Warm olive oil in a large skillet over medium heat. Add garlic and cook 30 seconds.
Stir in marinara, water, Italian seasoning, and red pepper flakes. Simmer 2–3 minutes.
Fold in meatballs and cook 5 minutes, stirring once, until heated through.
Combine drained spaghetti with the sauce and half of each cheese. Season with salt and pepper.
Spread mixture in the baking dish and sprinkle with remaining mozzarella and Parmesan.
Cover with foil and bake 20 minutes. Uncover and bake 8–10 minutes until browned and bubbly.
Rest 5 minutes, then garnish with parsley and serve hot.
Notes
Variation: Dollop ricotta between layers for a creamier bake, or swap in turkey meatballs for a lighter take. Make-ahead: Assemble up to 24 hours in advance, cover, and refrigerate; add 5–10 minutes to bake time. Leftovers reheat well; store chilled for 3 days or freeze up to 2 months.This recipe is an original creation inspired by classic Easy Baked Spaghetti and Meatballs flavors. All ingredient ratios and instructions are independently developed.