Chicken involtini with white polenta

Ingredients

• 2 large chicken breasts (400gm each)
• 12 thin prosciutto slices
• 2 buffalo mozzarella (250gm each), each sliced into 6
• 40 sage leaves (about 1 bunch)
• 100 gm butter, coarsely chopped
• To serve: lemon wedges
• White polenta
• 1 litre (4 cups) chicken stock
• 250 gm (1½ cups) white polenta (see note)
• 30 gm (½ cup) finely grated parmesan
• 40 gm butter, coarsely chopped