Preheat oven to 350°F. Spray an 8-inch square baking pan with cooking spray.

Spray a small sauté pan with cooking spray. Over medium heat, cook onions until soft, about 5 minutes.

While the onions are cooking, heat milk with vegetable broth over medium heat in a large saucepan. Bring to a boil. Slowly whisk in cornmeal, making sure there are no lumps. Reduce heat to low and continue stirring until thickened. Add sautéed onions and salt.

Pour half of the polenta into the prepared baking pan. Sprinkle mozzarella and Parmesan over top. Pour the rest of the polenta on top of the cheese. Bake at 350°F for 25 minutes.

In the last 5 minutes of baking, warm tomato sauce in a medium saucepan over medium heat.

Scoop out portions of polenta onto dinner plates and top with tomato sauce.