Put butter in sauce pan. A skillet will work as well if it can hold all the onions, it is also best if it does not have non stick coating.
Sautee the onions until they have reduced down and are caramelized. You want some to be sticking to the pan and starting to brown. This can take up to 45 minutes.
When they start to darken deglaze the pan with 1 cup of the beef stock. Repeat until the onions are a dark brown and not stuck to the pan. If you are using a skillet now is when you need to put them into the a pot.
Add in the rest of the beef stock, the chicken stock, thyme, salt and pepper. Adjust seasonings to taste.
Let simmer for at least 20 minutes. Taste and adjust seasonings again.
To serve place in individual small bowls or ramkins cover with a slice of French bread and swiss scheese. Toast or broil to melt cheese and toast bread.