Steam the broccoli until crisp tender, 3-4 minutes.
While the broccoli cooks, in a small bowl, whisk together the arrowroot and 2 tablespoons of water to create a slurry. Set aside.
In another small bowl, whisk together the peanut butter, soy sauce, rice vinegar, and honey.
Heat the sesame oil in a skillet over medium-high heat. Once oil is shimmering, add the bell pepper, onion, and garlic and cook, stirring until softened, about 3 minutes. Add the broccoli and cook, stirring for about 3 more minutes. Add the prepared sauce to the pan, stir, and then add the arrowroot slurry and stir well to let the sauce thicken, about 2 minutes. Garnish with the sesame seeds and serve.