Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F. Grease a 3-qt. gratin dish with 1 Tbs. of the butter.
• Put the cream, parsley, peppercorns, thyme, lemon peel, bay leaf, and garlic in a medium saucepan over medium heat. Bring to a gentle boil, and then turn off the heat and set aside, covered, for 30 minutes.
• Fill a large bowl with cold water, and add the lemon juice. Peel the potatoes, and using a sharp knife or mandoline, cut into 1/8-inch-thick slices. Put the potatoes in the lemon water, and set aside for 15 minutes.
• Drain the potato slices and pat dry. Layer the potato slices in the buttered gratin dish, sprinkling each layer with a little of the chopped shallot and seasoning each layer with 3/4 tsp. salt and pepper to taste. Continue to layer, gently pressing down on the potatoes to ensure even layers, until you have used all the potatoes and shallots. You should end up with about 7 layers.
• Strain the infused cream into another medium saucepan, season with 1/2 tsp. salt and 1/4 tsp. pepper, and bring to a boil over medium-high heat. Pour the hot cream evenly over the potatoes, pushing on the slices gently with a wooden spoon to distribute the sauce.
• In a small saucepan, melt the remaining 2 Tbs. butter, and pour it in an even layer over the potatoes. Put the gratin dish on a large rimmed baking sheet, and bake until golden-brown, 60 to 70 minutes. If the gratin begins to brown too quickly, cover with a sheet of oiled foil. Let rest for at least 20 minutes, and then slice and serve.