Preparation
Melt butter or oil in large saucepan over medium heat. Add onion and cook, stirring, until translucent, about 5 minutes.
Add 1 1/2 cups water and 1 1/2 cups chicken stock and bring to a boil.
Stir together 1 cup stock AND water with 1 1/2 cups cornmeal. Gradually stir into the boiling water; cook, stirring constantly, over low heat until the cornmeal is thickened, about 15 minutes.
Preheat the oven to 350 degrees. Lightly butter a shallow 2 quart baking dish. Pour half of the polenta into the baking dish. Smooth with a spatula. Top the layer of polenta with Gruyere, mozzarella and Parmesan cheeses. Spread the remaining polenta on top and sprinkle with the remaining cheese.
Bake until the top is browned and bubbly, 35 to 45 minutes. Let stand for 10 minutes before serving.