For the frico slab: finely chop the onion and coarsely grate the potatoes with 200 g of Montasio cheese. Blend the Montasio cheese, potatoes, polenta flour and onions with salt and pepper. Spread the mixture evenly in a rectangular container and refrigerate for 30 minutes. For the Montasio cream: after heating the milk, cut the remaining Montasio cheese and melt it in the milk, stirring until you have a smooth cream. Cover the mixture with plastic wrap and allow to cool. Cut the frico slab into rectangular portions and brown them in a hot pan. Place the slab in the center of the dish and garnish with the Montasio cream.