Preheat the oven to 180 degrees C. Grease a 23cm square baking dish.
In a large pot combine the milk and chicken stock. Bring to a boil over medium-high heat. When it is at a rolling boil gradually whisk in the polenta, making sure there are no lumps. Reduce heat to low and simmer, stirring constantly until thick, about 5 minutes. Remove from the heat and stir in the Parmesan cheese.
Pour the polenta into the prepared baking dish and spread spaghetti sauce over the top.
Bake for 10 minutes in the preheated oven or until sauce is bubbling.