For stuffing, in a medium saucepan cook onion in hot butter over medium heat until tender. Remove from heat. Stir in dried pears, walnuts, Gorgonzola cheese, and balsamic vinegar.
Untie meat; trim fat from meat. Spoon stuffing onto the flat side of one portion of meat. Top with second portion of meat; retie meat with heavy 100-percent-cotton kitchen string. For rub, in a small bowl stir together snipped rosemary, garlic, and pepper. Sprinkle rub evenly over meat; rub in with your fingers.
For a charcoal grill, arrange medium coals around a drip pan. Test for medium-low heat above pan. Place roast in a roasting pan on grill rack over drip pan. Cover and grill for 1-1/2 to 2 hours or until thermometer inserted into center of meat registers 150 degrees F, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium-low. Adjust for indirect cooking. Place roast in roasting pan; place pan on grill rack over the burner that is turned off. Grill as directed.)
Remove meat from grill. Cover meat with foil; let stand for 15 minutes. (Temperature of the meat will rise 10 degrees F during standing.) If desired, serve meat with grilled nectarine slices and garnish with rosemary sprigs.