Chop the onion and leek.
Heat the olive oil in a strong earthenware casserole dish - add the onion and leek and fry on a low heat until they are a golden brown colour (about 10 mins).
Add the garlic, parsley, sliced potatoes and a couple of pinches of salt - cover with the water and simmer until the potatoes are nearly cooked (this takes about 20 mins depending on thickness of potato slices)
After covering the fish pieces in flour add them to the casserole dish
Pour the white wine over the fish and cook for about 10 mins over a low heat until the fish is done