Heat half a litre of salted water, sprinkle the polenta into the water and cook for around ten minutes, stirring often. Pour into a greased tray and leave to cool. Sauté the garlic and parsley in a little oil, add the prawns and the wine. Reduce the wine by half and add the knob of butter rolled in flour and the drained peas. Salt and pepper to taste and leave to thicken slightly. Cut out 8 discs using a pasta cutter (or wide glass) and cut 4 of them again in the centre to obtain 4 rings, then place them on the larger discs to obtain little hollow baskets. Fill them with the prawn and pea sauce and serve.