Peel and finely dice the onion. Melt the butter in a saucepan. Then sauté the onion cubes while stirring until light yellow. Add green spelled flour and sauté for a while while stirring. Pour in the vegetable stock and whisk through everything with a whisk, making sure that there are no lumps. Bring the soup to a boil and cook without a lid over low heat for about 10 minutes, stirring occasionally. Pour in the cream, heat it up and season the soup with salt, pepper, sugar and nutmeg.