Cook the broccoli in boiling water for 2 minutes. Drain, rinse under cold running water and drain again.
Whisk together the chicken broth, soy sauce, mirin, cornstarch, sriracha, sugar and sesame oil in a medium bowl until dissolved.
Add the oil to a large (12-inch) non-stick skillet and heat on medium-high heat until it begins to shimmer. Add the chicken and cook 5-7 minutes, stirring frequently, until the chicken is cooked through and no longer pink.
Stir in the garlic and ginger and cook a few seconds until fragrant. Pour in the broth mixture and bring to a simmer. Cook 3-4 minutes, until the sauce is thickened. Add the broccoli to the pan and stir to combine with the chicken and sauce.
Sprinkle the green onion and sesame seeds over the chicken and serve.
Frozen broccoli florets can be used in place of fresh. Blanch them for 1 minute instead of 2 in step 1.
Serving suggestion: Serve with hot cooked rice or rice noodles.