Preheat the oven to 400°. Bring a large pot of salted water to a boil. Squeeze the lemon into a large bowl of water. Peel the celery root and cut it into 1/2-inch dice, adding it to the bowl of lemon water as you go.
Drain the celery root and add it to the boiling water. Boil until tender, about 7 minutes. Drain well and wipe out the pot.
In the same large pot, melt the butter. Add the leeks, chopped thyme and crushed red pepper and cook over moderate heat, stirring occasionally, until the leeks are softened, about 7 minutes. Add the wine and simmer over moderately high heat, stirring, until almost evaporated, about 4 minutes. Add the celery root and cream and simmer for 1 minute. Stir in 1 cup of the Parmesan and the parsley. Season with salt and black pepper. Transfer the mixture to two 10-by-15-inch baking dishes.
Sprinkle the remaining 1 cup of Parmesan over the gratins and bake for 25 minutes, or until bubbling around the edges and golden brown on top. Let stand for 10 minutes before serving.
The gratins can be prepared through Step 3 and refrigerated overnight. Bring the gratins to room temperature before proceeding.