Prep the oven and pan: Heat the oven to 400°F (200°C).
Lightly oil a 9x13-inch baking dish or line a sheet pan with parchment.
Make a quick panade: Combine breadcrumbs and milk in a large bowl. Let it sit for 2–3 minutes until it looks like thick oatmeal. This step keeps the meatballs moist.
Add aromatics and seasonings: Stir in egg, grated onion, garlic, Parmesan, parsley, Italian seasoning, red pepper flakes, salt, and pepper.
Mix in the meat gently: Add ground beef and pork.
Use your hands or a fork to combine until just mixed. Do not overwork the mixture or the meatballs will be tough.
Shape the meatballs: Scoop into 1.5-inch balls (about 2 tablespoons each). You should get 20–24 meatballs. Lightly oil your hands for easier rolling.
Arrange and bake: Place meatballs in the prepared dish with a bit of space between them.
Bake for 12–14 minutes, until they’re starting to brown and almost cooked through.
Add sauce: Remove from the oven and pour marinara around and partially over the meatballs. Spoon sauce on top to coat, but don’t drown them.
Add cheese and finish baking: Sprinkle mozzarella evenly over the top. Bake another 10–12 minutes, until the cheese is melted and bubbling and the meatballs reach 165°F in the center.
Broil for color (optional): Broil 1–2 minutes to brown the cheese lightly.
Watch closely.
Rest and serve: Let the dish rest 5 minutes so the sauce settles. Garnish with basil. Serve with pasta, crusty bread, or on toasted sub rolls.