Heat the fat, sweat the flour in it until golden. Deglaze with milk and stock. Simmer for about 5 minutes while stirring. Melt the cheese in the soup. Season to taste with salt, pepper and nutmeg.
Shape the onion mince into small dumplings with wet hands and simmer with the peas in the soup for another 5 minutes. Chop the chives and add to the soup before serving.