2Bring chicken stock and butter to a boil in a large saucepan over high heat. Whisk in polenta in a steady stream, and reduce heat to medium. Simmer, stirring constantly until the polenta has thickened, and the grains are tender, 20 to 30 minutes. Be careful while stirring the polenta, it can cause burns as the bubbles pop and send hot polenta flying.
3Once the polenta grains are tender, stir in the garlic, roasted pepper, prosciutto, mozzarella cheese, Parmesan cheese, Gruyere cheese, and sage leaves; cook and stir an additional 5 minutes. Season to taste with salt and pepper, and pour into prepared cake pan. Place into refrigerator, and chill until firm, about 1 hour.
4Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with a sheet of aluminum foil, and lightly oil.
5Unmold the polenta onto a cutting board, and cut into 8 wedges. Arrange the slices onto the prepared baking sheet so the slices are not touching.
6Bake in preheated oven until the edges begin to brown, and the polenta is hot in the center, 10 to 15 minutes.