Preheat the oven to 180 C, 160 Fan. Cut the chard leaves from the stems and chop the stems into short lengths. Cook briefly in boiling lightly salted water until crisply tender.
Add the leaves and cook briefly till they just wilt. Drain the vegetables well and tip into a buttered shallow dish.
Put the mustard in a bowl and stir in the cream and a grinding of salt and pepper. Pour over the chard.
Cover with grated Parmesan and bake for 20-30 mins until the top has a light crust the colour of honey.