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.