Preheat oven to 350 F. In a saucepan, melt butter over medium heat. Add garlic and shallot and saute until translucent, about 2 minutes. Add the cream and bring to a simmer for 5 minutes. Season with salt and pepper, strain through a fine sieve and set aside. Peel the potatoes and slice into thin rounds with a Japanese mandolin. In a 4 x 6-inch baking dish, arrange two layers of potato slices, scatter with a third of the onion, thyme and Gruyere cheese. Cover with a little of the seasoned cream; repeat this three times, ending with a layer of potatoes. Cover with the rest of the cream. Cover with aluminum foil and bake for 40 minutes. Remove the foil and bake another 10 minutes, or until browned on top and easily pierced with a pairing knife. Keep warm. 

Fingerling potatoes:
Step 1:  Preheat oven to 350 F. Run fingerling potatoes with melted butter; season generously with salt and pepper. Arrange on a sheet pan and bake for approximately 30 minutes, or until easily pierced with a paring knife. With a paring knife, cut an oval shape out of the top of potatoes; carefully hollow out the inside of the potato with a small melon baller.

Step 2: Cut peeled potatoes into quarters; place in a medium-size pot, cover with water, and add salt; bring to a boil, then reduce heat to a simmer and cook approximately 20 minutes, or until potatoes are easily pierced with a paring knife. Strain through a colander and allow potatoes to steam dry for 2 minutes. Run the potatoes through a food mill with the cold butter, fold in the heated cream, truffle butter, salt, white pepper and chives. Transfer to a piping bag with star tip and keep warm.

Step 3: Preheat oven to 350 F. Pipe the Potato Filling into fingerling potatoes; bake until filing is lightly browned, about 12 minutes.

Creamy truffle vinaigrette:
Step 1: Place vinegar, shallots and salt in a small bowl; let sit 15 minutes. Add the black truffle puree and slowly add the canola oil in a steady stream while whisking constantly. Add the black truffle oil and freshly ground pepper.