In a bowl or large measuring cup, whisk together the sauce ingredients. Start with the soy sauce, rice vinegar, and sesame oil, then add the cornstarch and stir well. Then add the chicken stock, brown sugar, sriracha, garlic, and ginger. Set aside.
n a skillet over medium high heat, heat 1 tablespoon of oil, add the chicken and season it with black pepper. Cook the chicken until it’s done (about 3-4 minutes). Remove from the skillet onto a plate, and set aside.
To the same skillet, add the remaining oil and add the onion and mushrooms. Saute for 2 minutes.
Add the broccoli and cook for 2 more minutes. Add a splash of water (this will help soften the broccoli and at the same time you can deglaze the pan). Cook until the broccoli is tender but still crunchy.
Reduce the heat to medium-low, and pour the sauce ingredients to the skillet (give them a quick mix before adding as the cornstarch can sink to the bottom), simmer the vegetables with the sauce for 2-3 minutes.
Then add the chicken and cook until the chicken is just warmed up and coated in the sauce. Remove from heat, and serve over jasmine rice or noodles. Garnish with toasted sesame seeds.