Heat gas grill to medium-high or coals in a charcoal grill to medium-hot. Lightly coat grill rack with oil or cooking spray.
In a medium-size bowl, blend mayonnaise, ketchup, relish, 1 tablespoon of the dried onion and sugar. Cover and refrigerate.
In a large bowl, mix together ground chicken, remaining tablespoon dried onion, Worcestershire sauce, garlic powder and pepper. Form into 12 equal-size patties.
Grill burgers for 2 to 3 minutes per side or until temperature registers 160 degrees F on an instant-read thermometer. Remove to a plate.
To serve, set aside 6 bun tops. Spread each bun bottom with 1 tablespoon sauce. Stack each with a burger and 2 tablespoons shredded lettuce. Top each with a bun half; repeat stacking with sauce, burger and lettuce. Top each with a reserved bun top. Serve with remaining sauce.