In a large lidded casserole, heat a glug of oil, add the onion and fry for 15 minutes until soft and starting to brown. Add the garlic and leeks and fry for 5 minutes, then add the sherry (or wine) and stock.
Add the herbs and a grating of nutmeg, then season. Put the lid on and gently braise the leeks for 30 minutes until tender.
Stir in the cream, season to taste, then serve sprinkled with chopped parsley and a little extra nutmeg.
