This is one hearty meal. You have the lamb as your meat and protein, the potato-apple gratin as a side dish, and brussels sprouts as your vegetables. Your friends and family won't be disappointed when you serve this at your holiiday dinner! Chef Marcus Samuelsson recommends you pair it with Penfolds Bin 8 Cabernet Shiraz.
Preheat the oven to 450 degrees.
Put the peppercorns in a small saucepan, add water to cover, and bring to a boil; drain. Add cold water to cover, bring to a boil, and drain again; repeat 2 more times. Spread the peppercorns on paper towels and let dry thoroughly.
Put the peppercorns in a mini processor or blender and process until coarsely crushed. Combine in a small bowl with the egg yolk, mustard, 1 teaspoon of the olive oil, the Parmesan, bread crumbs, thyme, and rosemary and blend to a coarse paste.
Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat until almost smoking hot. Add the racks of lamb and sear for about 3 minutes on each side, or until lightly browned. Transfer to a platter and let cool.
Season the lamb with salt and pepper. Spread the herb mixture over the meaty part of the lamb. Place the lamb, fat side up, on a rack in a roasting pan. Roast for 15 to 20 minutes, or until the internal temperature registers 125 degrees on an instant-read thermometer, for medium-rare. Remove from the oven and let rest for 10 minutes.
Combine the wine, stock, vinegar, thyme, and lemon zest in a medium saucepan, bring to a simmer, and simmer for 20 minutes, or until reduced to 1 cup. Remove from the heat and cover to keep warm.
Just before serving, reheat the sauce if necessary, and whisk in the lemon juice and butter. Season with salt and pepper to taste.
To serve, cut the racks into individual chops and arrange 3 or 4 chops on each plate. Pass the sauce at the table.
Preheat oven to 400 degrees. Butter 13x9x2 inch glass or ceramic baking dish.
Heat the cream and bring to a boil. Add the rosemary to the cream, and steep for 15 minutes.
Meanwhile, melt 6 tablespoons butter in a large nonstick skillet over medium heat. Add onions, thyme, and 2 tsp salt; sauté until onions are translucent, about 10 minutes. Increase heat to medium-high; sauté until onions are tender and begin to color, about 8 minutes longer. Remove from heat. Add remaining 6 tbsp butter and cream skillet; stir and swirl skillet to combine. Bring to boil. Cool onion mixture to lukewarm.
Combine potatoes, apples, remaining 2 tsp salt, and onion mixture in large bowl; toss gently to blend. Transfer to prepared baking dish, spreading evenly. Sprinkle cheese on top.
Cover dish with parchment paper, then cover with foil, shiny side down. Bake gratin for 1 hour and 10 minutes. Let gratin stand 15 minutes before serving.