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Heat oven to 200C/180C fan/ gas 6. Strip the chard leaves from the stalks, then cut the stalks into sticks. Bring a pan of water to the boil and cook the stalks for 3-4 mins until starting to soften. Then throw in the leaves for a few moments too so that they just wilt. Drain well.
Mix the cream with the mustard, then toss through the chard with most of the gruyère. Grease a medium gratin dish, spread the chard mix over, then scatter with the remaining gruyère and the parmesan. Bake for 30 mins until bubbling and golden. Serve straight from the dish.