How to Make It
Preheat the oven to 350°. Rub the roast with 1/2 tablespoon of the oil and season with salt and pepper. Heat a large ovenproof skillet until very hot. Add the roast and cook over high heat, turning occasionally, until well-browned all over, about 12 minutes. Remove from the heat and add the butter, garlic and thyme. Spoon the butter mixture over the meat and roast, basting, for about 20 minutes, or until an instant-read thermometer inserted in the center registers 125°. Let rest for 20 minutes.
In a medium saucepan of boiling water, blanch the brussels sprout leaves for 20 seconds. Drain and pat dry. In a skillet, heat the remaining 1 tablespoon of oil. Add the chestnuts and cook over moderate heat, stirring, until golden, 3 minutes. Add the pear and sprout leaves and cook, stirring, for 1 minute. Sprinkle with the Sichuan peppercorns and season with salt.
Untie the roast and carve it across the grain into thin slices. Garnish with the chestnuts, pear and brussels sprouts.