Peel and coarsely chop the potatoes and the celery root. Put potatoes and celery root in separate pots and add salt to each.
Boil the potatoes and celery root until soft, and then drain them.
Pour the heavy cream into a pot and bring it to a boil. Add the butter, saffron, garlic, and 1 cup of the gruyere. Cook about 5 minutes over moderately low heat until the cream mixture is bright yellow.
Puree drained celery root in food processor, gradually adding yellow cream mixture.
Mash the drained potatoes to a rough texture.
Add pureed celery root to mashed potatoes. Add the rest of the gruyere and mix together. Season with salt and pepper.
Butter a 13 by 9-inch gratin dish and put the puree in it. Sprinkle with the parsley.
Bake at 400 degrees for about 15 minutes.