Spray a wire baking rack and grill with cooking spray; place rack on grill and then preheat to medium-high.

Slice each plantain on the bias into 1/2- to 1-inch-thick pieces and place in a bowl. Toss with lime juice. Combine chili powder and cinnamon; dust over plantains and toss.

Place plantains on wire rack and cook slowly, turning often, until slightly crisp, about 10 minutes. Transfer to a bowl or platter and sprinkle with herbs or green onion, if using.