Cook plantain in boiling water 15 minutes or until very tender. Drain; place in a large bowl. Mash plantain with a fork until smooth; cool 5 minutes. Add green onions and next 7 ingredients (through chicken). Divide mixture into 6 equal portions, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 4 minutes on each side or until a thermometer registers 165°. Remove from grill, and let stand 5 minutes.
Place buns, cut sides down, on grill rack; grill 1 minute or until toasted. Place 1 patty on bottom half of each bun; top each serving with 1 red onion slice, 2 tomato slices, and top half of bun.