- Unsalted butter, for baking dish
- 1 Tbsp olive oil
- 1/2 cup finely chopped shallots
- 2 cloves garlic, minced
- 1 tsp chopped fresh thyme
- 2 cups heavy cream
- 1 tsp Dijon mustard
- 2 tsp salt
- 1/4 tsp freshly ground pepper
- Juice of 1 lemon
- 3/4 lb russet potatoes, peeled and sliced 1/16" thick
- 3/4 lb sweet potatoes peeled and sliced 1/16" thick
- 1 1/2 lb celery root, peeled and sliced 1/16" thick
- 4 oz Gruyere cheese, grated
- 2 oz Parmigiano-Reggiano cheese grated
- 2 tsp chopped fresh flat-leaf parsley
Preheat oven to 375 degrees. Butter a 13″x9″ baking dish.
In a saucepan over medium heat, warm olive oil. Add shallots and cook until translucent, about 3 minutes. Add garlic and thyme and cook until fragrant, about 30 seconds. Add cream, mustard, salt and pepper and whisk until just blended. Remove from heat and let cool slightly.
Arrange a layer of potato and celery root slices, slightly overlapping, in prepared baking dish. Sprinkle with one-third of cheese mixture. Pour over one-third of cream mixture. Repeat layering 2 more times, ending with cheese. Pour any remaining over gratin.
Cover dish loosely with foil and bake for 30 minutes. Remove foil and continue baking until top is golden and vegetables are tender when pierced, another 30-45 minutes. Let stand for 10 minutes and garnish with parsley.