HERBED CRUMBS: 1 cup fresh white breadcrumbs 5 tablespoons unsalted butter, room temperature 2 tablespoons chopped fresh parsley 2 teaspoons chopped fresh tarragon or 3/4 teaspoon dried 1/4 teaspoon chopped fresh rosemary or pinch of dried LAMB: 2 2 to 2-1/4 lb. racks of lamb, boned, trimmed, each cut in 3 pieces 1 tablespoon unsalted butter 1 tablespoon olive oil FOR PUREE: Melt butter in heavy large skillet over low heat. Add onions; cover and cook until tender, stirring occasionally, about 25 minutes. Bring small pot of water to boiL; add potato and cook 3 minutes. Using slotted spoon, transfer potato to skillet with onions. Add cream, mushrooms, garlic and rosemary. Simmer until cream is reduced to 1/4 cup, stirring occasionally, about 15 minutes. Transfer to processor, puree. Season with white pepper and salt. FOR CRUMBS: Process breadcrumbs, butter, parsley, tarragon and rosemary in processor until well blended, using on/off turns. Transfer to plate. FOR LAMB: Season lamb with salt and pepper. Melt butter with oil in heavy large skillet over high heat. Add lamb and cook until brown, about 4 minutes per side for medium-rare. Transfer to plate; cover with foil and let rest 10 minutes. Preheat broiler. Pat herbed crumbs neatly onto top of each lamb piece. Place in broiLer pan. Broil lamb 6 inches from heat until crumbs are golden brown, watching closely to avoid burning, about 2 minutes. Reheat onion puree in medium saucepan over medium-low heat, stirring occasionally. Spoon a few tablespoons puree onto each of 6 plates. Cut 1 thin slice from each lamb piece and arrange lamb piece and lamb slice atop puree. Serves 6. lamb 30