In a liquid measuring cup, combine the soy sauce, sesame oil, honey, starch, garlic and red pepper flakes (if using). Whisk until blended and set aside.
Warm the sesame seeds in a large cast iron or stainless steel skillet over medium heat, stirring constantly, until fragrant and turning lightly golden, about 1 minute. Transfer them to a plate and set them aside, then return the skillet to the stove.
Warm the oil over medium heat until shimmering. Add the onion and carrots and a pinch of salt. Raise the heat to medium-high and cook, stirring every 30 seconds or so, until the onions have softened, about 4 to 6 minutes.
Add the bell peppers and broccoli and stir to combine. Pour in the water and immediately cover the pan. Cook, uncovering to stir every minute or so then covering again, until the broccoli and bell pepper are fork-tender and browning along the edges, about 7 to 9 minutes.
Uncover and remove the skillet from the heat. Immediately pour in the prepared sauce and stir well to coat the veggies. Taste, and if it needs a little more flavor, stir in a pinch of salt. Serve as desired, sprinkled with toasted sesame seeds and green onion. Leftovers keep well in an airtight container in the refrigerator for up to 4 days.
