Lay the trout in a poacher or roasting pan. Pour the wine in, and add enough water to cover. Drop in the onion, bay leaf and peppercorns. Salt. Cover, and bring just to a boil. Turn off the heat and leave to cool to room temperature. Transfer the fish to a serving platter. Let sit 30 minutes, then drain off any excess liquid. Pat dry. Remove the skin from the fish, leaving head and tail intact.
Prepare the cream mixture by beating the mascarpone with the chives, lemon, salt and pepper. Cut down the back of the fish and open out the filets. Remove the bones. Spread most of the cream mixture inside and sandwich the filets back together. Spread the cream in a thin layer of the remaining cream over the flesh of the fish. Lay over cucumber slices to cover the flesh completely, like fish scales. Lay a chive flower over the eye. Serve.