3. 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.