Heat oil in a nonstick or cast iron pan. Add meatballs and cook for about 1 minute per side.: The pan should hiss when the small amount of olive oil meets it, creating a shimmering surface that signals it is ready. I listen for a faint sizzle and watch for the oil to thin and ripple. A properly heated pan prevents the meatballs from sticking and supports a quicker, even browning. If the pan is not hot enough you will notice soggy edges instead of a golden crust, so give it a few extra seconds before adding anything.
Add marinara sauce and simmer until meatballs are heated all the way through.: When the meatballs hit the pan they should sizzle audibly, and you will smell the meat warming and the oil toasting. As you turn each meatball, look for areas of golden brown, which indicate caramelization and added flavor. This quick sear enhances texture and builds savory depth that the sauce will absorb. Avoid crowding the pan, because crowded meatballs steam rather than sear, leaving them soft and less flavorful.
Sprinkle basil and cheeses on top and continue to cook over low heat until cheese is melted. (Or you can stick the pan under the broiler to melt the cheese.): Pour the full jar of marinara sauce over the browned meatballs, and you will see the sauce come alive with tiny bubbles as it reaches a gentle simmer. The steam will carry a rich tomato aroma, and the sauce will start to thicken slightly as flavors meld. Simmering heats the internal portion of the meatballs and allows flavor exchange between the sauce and meat. A common mistake is to rush this step with too high heat, which can cause the sauce to break or stick to the pan, so keep it at a gentle simmer and stir occasionally.
Serve over pasta or with garlic toast slices.: Once the meatballs are hot through, scatter the chopped basil , grated Parmesan cheese , and shredded mozzarella cheese over the top. You will notice the basil release a fragrant herbal scent as it warms, and the cheeses will slowly collapse into a creamy layer, bubbling faintly. Cooking over low heat ensures the cheeses melt evenly without separating, creating a glossy, cohesive topping. Avoid turning the heat up to speed melting, because that can make the cheese oily and grainy.
Or you can stick the pan under the broiler to melt the cheese.: If you choose the broiler finish, move the skillet to the oven and broil briefly until the cheese develops faint golden spots and begins to bubble vigorously. The visual cue is a lightly bronzed top with bubbling edges, which adds a pleasing contrast in flavor and texture. Be vigilant, because under a broiler the cheese can go from perfect to burnt within seconds. Use an oven mitt and stand close enough to watch, but not too close to risk burns from hot steam.
Serve over pasta or with garlic toast slices.: Transfer the steaming pan or plated portions onto cooked pasta or beside slices of garlic toast . The steam from the meatballs will marry with the pasta, and the melted mozzarella cheese will stretch and cling in delightful strands. Look for a glossy sauce coating and listen for the soft clink of a fork through cheese when you serve. A frequent oversight is plating too soon, before the cheese settles; let it rest a couple of minutes so the layers hold together when you portion.