Slice. Toss the chicken in the freezer for 10 minutes, then use a sharp knife, and cut 1/4" thick slices against the grain (find the tiny white muscle fibers and slice against them, rather than parallel to them).
Marinade. In a large bowl, combine the marinade ingredients. Add the chicken and toss to coat. Place the bowl in the fridge for at least 30 minutes before you begin.
Sauce. Whisk together the water (3 tablespoons), oyster sauce (1 tablespoon), soy sauce (1 tablespoons), rice vinegar (1 tablespoon), and sesame oil (1 teaspoon) in the small bowl. Add the cornstarch (1 teaspoon) (if using) and whisk until dissolved.
Chicken. Heat a large pan or wok over high heat and add 2 tablespoons of oil. Once shimmering add the chicken in a single layer. Make sure not to overcrowd the pan, you may need to do this in batches. Once the chicken is golden brown (should be 30 seconds-1 minute), flip and repeat on the other side. Transfer cooked chicken to a clean plate and set aside.
Veggies. Reduce the heat of your pan to medium, and add the remaining tablespoon of oil. Once shimmering add the green beans (1½ cups), mushrooms (1 cup), and onion (1 cup). Cook until softened, about 8-10 minutes.
Bringing it all together. Add the sauce, chicken, and any accumulated chicken juices back to the pan. Toss everything together and continue to cook until the sauce has thickened to your liking, about 2-3 minutes.
Serve immediately over rice or cauliflower rice with green onions and sesame seeds. Enjoy!