Saute the onions with the garlic in the butter until translucent, add vegetable broth and milk, bring to a boil, add the cornmeal, season with salt and pepper. Let the polenta swell in the pan with a lid or in a bowl with a lid in the preheated oven at 120°C for 1 hour.
Stir the mascarpone and Schabziger into the polenta. Place cake ring in an ovenproof dish or on the baking sheet, pour in polenta and smooth out, let cool. Remove ring. Sprinkle Gruyère over polenta, drizzle with butter. Cut polenta into 12 pieces. Bake briefly on grill setting.