1. Preheat the oven to 350°. Using 1 teaspoon of the butter, grease a 9-inch glass pie plate and a 12-inch square of foil.
2. In a medium skillet, melt 1 teaspoon of the butter. Add the leek and cook over moderately high heat until softened but still green, about 2 minutes. Transfer the leek to a plate.
3. Melt the remaining 1 teaspoon of butter in the skillet and add the bread crumbs. Cook, stirring, until coated and lightly browned. Transfer the crumbs to a plate and toss with the Gruyère.
4. Layer half of the Jerusalem artichoke slices in the prepared pie plate and season with salt and pepper. Cover with half of the leek slices and half of the sweet potato slices, reserving the nicest slices for the top. Season with salt and pepper and repeat with the remaining Jerusalem artichokes, leek and sweet potato. Pour the cream evenly over the top.
5. Cover the pie plate with the foil, buttered side down, and bake for 30 minutes, or until bubbling and the vegetables are just tender. Remove the foil and bake for 20 minutes, or until lightly browned. Sprinkle the crumbs on top and bake for 20 minutes longer, or until golden. Let stand for 15 minutes before serving.