For the potato and Jerusalem artichoke gratin with mustard and lemon, first preheat the oven to 180 degrees. Boil the potatoes and Jerusalem artichokes separately in salted water for 8 to 10 minutes, then drain, let cool slightly and peel.
Heat oil in a frying pan. Add onions, garlic and parsley and sauté over low to medium heat.
Add whipping cream, mustard and lemon zest and briefly bring to a boil. Season vigorously with salt and freshly ground pepper.
Cut large potatoes and Jerusalem artichokes in half, leave smaller ones whole. Spread evenly in a buttered baking dish, pour the mustard sauce over, sprinkle with the cheese and bake for 30 minutes until golden brown.
Arrange the potato and Jerusalem artichoke gratin with mustard and lemon and serve.