Preheat oven to 400°.
To prepare relish, arrange pears in a single layer in a 13 x 9-inch baking dish. Arrange onion slices in a single layer in another 13 x 9-inch baking dish; lightly coat onion slices with cooking spray. Bake pears and onion slices at 400° for 40 minutes or until tender, turning once. Cool; chop.
Combine 2 teaspoons rind and next 7 ingredients (through 1/4 teaspoon black pepper) in a large bowl; add chopped onion and pears, tossing gently to combine. Stir in nuts just before serving.
To prepare lamb, combine 1 tablespoon lemon rind and next 10 ingredients (through onion wedges) in a food processor; process until finely chopped. Roll roast, and secure at 3-inch intervals with twine. Spread the onion mixture over lamb; cover and refrigerate 8 hours or overnight.
Preheat oven to 450°.
Place lamb on the rack of a broiler pan coated with cooking spray; place rack in pan. Bake at 450° for 20 minutes. Decrease oven temperature to 300° (do not remove lamb from oven); bake an additional 1 hour or until thermometer registers 145° (medium-rare) to 160° (medium). Let stand 10 minutes before slicing. Serve relish with lamb.