1. Preheat the oven to 350°F, and position the rack in the middle. Line a sheet pan with parchment paper or foil.

2. In a bowl, mix half of the onion, half of the garlic, oats, 1/4 cup of Parmesan, 2 tablespoons of parsley, egg, milk, and 1/2 teaspoon of salt. Add chicken, and combine.

3. Form the mixture into one-inch meatballs (about 24), and place on the prepared sheet pan. Bake for 15 minutes, turning after 10 minutes, until golden brown.

4. While the meatballs bake, heat olive oil in a pan, and sauté the remaining onion, garlic, and mushrooms over medium-high heat for 15 minutes, or
until softened and caramelized. Lower the heat to medium if the vegetables start to burn.

5. Sprinkle flour over the mushroom mixture, stir, and cook for 30 seconds. Add chicken stock and Marsala wine. Boil until thickened.

6. Add the meatballs to the sauce, and simmer on low for 10 minutes, turning occasionally — season with extra salt, if needed, and the remaining parsley.

7. Bring 3 cups of water and 1/2 teaspoon of salt to a boil. Whisk in the polenta, reduce heat to low, and simmer for 10 minutes, stirring occasionally. Add the remaining 1/2 cup of Parmesan.

8. Spread the polenta on a platter or dish, and top with the meatballs and sauce.