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Heat oven to 200C/180 fan/gas 6. Boil the potatoes for 15-20 mins until tender, then drain. Reserve 3 tbsp corn and blitz the rest (with its juice) using a hand blender or a food processor until completely smooth, then mash into the potatoes. Cook the spinach following pack instructions – if you have a microwave, choose this method.
Put the leeks in a pan with the 300ml milk and the butter. Cook gently, part-covered, for 8 mins until the leeks are tender. (Keep an eye on things as milk can easily boil over.) Mix the 3 tbsp milk with the flour and mustard, then stir into the leek mixture – keep stirring until thickened. Take off the heat and stir in three-quarters of the cheese.
Squeeze as much liquid as you can from the spinach, then arrange on the base of 2 gratin dishes. Place a fish fillet on top of each, then spoon over the leek & cheese sauce. Top with the sweetcorn mash. Mix the remaining corn and cheese, and scatter on top. Place the dishes on a baking tray and cook for 25 mins until the fish flakes when tested and the top is golden.