Combine first 12 ingredients in a large bowl. Divide mixture evenly into 8 portions, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 5 minutes on each side or until done. Place buns, cut sides down, on grill rack; grill 1 minute or until toasted. Spread 1 teaspoon mustard on each bun top. Place 1 patty on bottom half of each bun; top each serving with 2 pickle slices, 1 lettuce leaf, 1 tomato slice, and top half of bun.