Preheat oven to 425°.
Place bread in a food processor; pulse 10 times or until coarse crumbs form to measure about 1 1/4 cups.
Combine the breadcrumbs, nuts, rind, parsley, juice, and 1/2 teaspoon salt in a small bowl.
Combine chives, mint, mustard, and garlic in a small bowl.
Sprinkle lamb with 1/4 teaspoon salt. Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add lamb racks; cook 2 minutes on each side or until browned. Spread half of the mustard mixture over meaty portion of each lamb rack. Carefully pat half of the breadcrumb mixture into mustard mixture on each lamb rack.
Place the lamb on a broiler pan coated with cooking spray. Bake at 425° for 35 minutes or until meat thermometer registers 140° (medium-rare) to 155° (medium). Place lamb on platter; cover with foil. Let stand 10 minutes before serving (temperature will increase 5° upon standing). Slice each rack into 4 pieces (2 ribs per piece). Garnish with mint sprigs, if desired.