Preheat the oven to 400 degrees . Slice between the light- and dark-green parts of the scallions. Quarter the white and light-green sections lengthwise to form long ribbons, and thinly slice the dark- green parts into rounds. Using a peeler, cut the carrot into thin strips
Place four 18-inch-long sheets of heavy-duty foil on a work surface. Place one piece of chicken in the center of each; top with the peppers, carrot and scallion ribbons.
In a small bowl, whisk the soy sauce, brown sugar, vinegar, sesame oil and sriracha, if using. Drizzle over the chicken and vegetables. Bring the edges of foil up to meet in the center, leaving roughly 2 inches of space above the chicken and vegetables so the food can steam. Crimp the edges tightly to seal. Transfer the packets to a baking sheet. Bake until the chicken is cooked through and the vegetables are crisp- tender, 12 to 15 minutes.
Meanwhile, in a small, dry skillet, stir the sesame seeds over medium-high heat until golden, 5 to 6 minutes.
When the chicken is done, carefully open the packets. Using tongs, slide the contents of each packet onto separate plates. Drizzle with any accumulated juices. Sprinkle each serving with 1 tsp. sesame seeds and garnish with the scallion greens.