six 12-inch metal or wooden skewers (soak wooden skewers in water for 20 minutes)
Preheat a grill on medium-high heat.
Warm the teriyaki sauce and sugar together in a small saucepan over low heat and stir until the sugar dissolves.
Cut the bell peppers into 1 1/2-inch pieces. Cut the white and light green parts of the spring onion into 1-inch pieces (discard the dark greens or reserve for another use).
Thread the vegetables on six skewers, 2 pieces of pepper for every 1 piece of onions. Grill the vegetables until charred in spots and lightly cooked, about 6 minutes. Baste both sides of the skewers with the glaze a few times, turning often so that it chars in spots, about 1 minute. Transfer the skewers to a platter and drizzle the remaining glaze on top. Sprinkle with sesame seeds and serve.