Wash hands with soap and water. Preheat the oven to 375 degrees. Peel and chop the onion. Grease a casserole dish.

In a skillet, heat a little oil over medium-high heat. Add the onion and cook until soft, about 4 minutes. Add the garlic and sauté until fragrant, 1 minute more. Set aside to cool to room temperature.

In a medium bowl, combine the cooled onion mixture with the Parmesan, egg, breadcrumbs (omit for gluten-free), and ground beef; mix until combined. Using a spoon, scoop the meat mixture into 1-inch balls and arrange them evenly in the prepared casserole dish. Bake until the meatballs are fully cooked, 20 to 25 minutes. Pour the marinara sauce over the meatballs, toss to coat and then return the pan to the oven until the sauce is bubbly, 8 to 10 minutes more.

Meanwhile, in a medium heavy-bottomed saucepan, bring 1 cup milk and 3 cups water to a simmer over medium heat. Whisk in the cornmeal and cook, whisking occasionally to prevent it from sticking to the bottom of the pan, until the cornmeal absorbs all of the liquid, 15 to 20 minutes (be careful, it may splatter).

Stir the butter and parmesan into the polenta and divide into 4 bowls. Top with meatballs and sauce. This recipe makes 4 servings. Refrigerate leftovers within 2 hours.