Peel the onions and cut them into thin slices.
Melt the butter in a pan.
Add the onions and slowly let them roast until they are golden.
When the onions are transparent, soft and sweet tasting, you add the stock.
Add salt and pepper to tast.
Let it simmer for 20 minutes.
The soup is now ready to serve.
In France it is normal to roast some bread and put it into the soup along with some cheese.