Melt half the butter in a saucepan and fry onions until transluscent. Remove onions and set aside.
Melt remaining butter and add flour. Stir constantly to form a roux. Gradually add stock and bring to a boil, then return onions to the saucepan.
Place the egg yolk in a bowl and add a ladleful of hot soup, whisking. Gradually whisk the mixture back into the hot soup. Add the cream and cheese and stir into the soup until melted. Season with salt, pepper and lemon juice.
Carefully warm the brandy in a ladle over a candle flame. Light the brandy with a long match and pour over the soup to flambe it. Serve!