750ml good vegetable stock 

Olive Oil

Butter

Salt & Pepper

(You can use beef or chicken stock, but it's lovely as described & veggie friendly)

Method
Finely slice the onions (I use a food processor) and allow them to sweat slowly in a good slug of olive oil and a generous knob of butter over a low heat for at least an hour stirring occasionally. Don't salt the onions, adjust the seasoning after the stock has been added. The onions should gently caramelise and not be allowed to burn. Pour on stock and simmer for 20 minutes.

The soup will improve if kept until the next day.