1. Preheat oven to 450°.
2. Pierce potatoes with a fork, and coat lightly with cooking spray. Bake at 450° for 50 minutes or until tender. Remove potatoes from oven, and cool slightly. Cut a lengthwise slit in each potato that goes to, but not through, the other side, and squeeze the ends to loosen potato flesh.
3. Combine mayonnaise and next 6 ingredients (through garlic) in a medium bowl. Stir in chicken and relish. Divide the chicken mixture evenly among potatoes. Top each potato with 1 tablespoon onions.