To make the stock, gently simmer the prosciutto hock with the celery, one of the onions cut in half, the stalks of the mint, the carrots, pea pods and water to cover by 2.5 cm (1 in). This will take 1-1.½ hours.
Chop the remaining onion finely.
In a large, heavy saucepan, fry the chopped onion gently in the olive oil and the butter until light brown. Add half the mint leaves and