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Heat oven to 200C/180C fan/gas 6 and heat the oil in a large frying pan. Add the leeks and some seasoning, and cook for 10 mins until really soft. Stir in half the parsley and spoon into 2 gratin dishes.
Divide the fish between the 2 dishes, mix the mustard into the cheese sauce and spoon over the top of the fish. Place on a baking tray and cook for 15 mins – you can put the bread in the oven to warm through for the last few mins if it is a little stale. Remove the bread from the oven and turn the grill on to brown the tops of the gratin for 2 mins, then remove from the oven. Scatter the gratin with the remaining parsley and serve with chunks of bread to mop up.