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 45
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).