Combine ground beef, breadcrumbs, Parmesan, parsley, egg, garlic powder, salt, and pepper in a bowl; mix until just combined and uniform, about 30 seconds.

Shape the mixture into 1-inch meatballs using wet hands to keep meat from sticking; aim for about 16 evenly sized balls for uniform cooking.

Brown meatballs in a large skillet with olive oil over medium-high heat; cook 2–3 minutes per side until golden brown.

Remove browned meatballs to a plate, leaving any flavorful drippings in the skillet; set aside as you build the sauce.

Sauté chopped onion and minced garlic in the skillet over medium heat until onions turn translucent and garlic is fragrant, about 3–4 minutes.

Simmer crushed tomatoes, oregano, and red pepper flakes (if using) in the skillet; bring to a gentle bubble, stirring occasionally, for 5 minutes.

Nestle browned meatballs into the sauce, submerging them halfway; cover and simmer over medium-low heat for 10 minutes to cook through.

Add spaghetti and 2 cups water to the skillet; press pasta gently into sauce until half submerged, stirring to prevent sticking.

Cook uncovered for 8–10 minutes, stirring occasionally, until spaghetti is al dente and sauce has thickened to a silky consistency.

Garnish with torn fresh basil, a sprinkle of Parmesan, and a drizzle of olive oil for a bright, aromatic finish; serve your one-pan masterpiece hot.