1. Combine first 9 ingredients in a large bowl and gently mix with hands until just combined. Form meat into 8 equal patties (about 3-1/2 inches in diameter by 3/4 inch thick) and chill on a plate 15 minutes.
2. Preheat grill. Grill burgers on lightly oiled rack over moderate to moderately low heat, turning once, until golden brown, and cooked through, about 10 to 12 minutes. If desired, grill buns cut side down until golden brown, about 2 minutes.
3. Put burgers and accompaniments on bottom of buns and top with remaining bun halves. Makes 8 burgers.