Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch square baking dish.
Bring milk and chicken stock to a rolling boil in a large pot over medium-high heat. Gradually whisk in cornmeal, making sure there are no lumps. Reduce the heat to low and simmer, stirring constantly, until thick, about 5 minutes. Remove from the heat and stir in Parmesan cheese.
Pour polenta into the prepared baking dish and spread spaghetti sauce over top.
Bake in the preheated oven until sauce is bubbling, about 10 minutes.