Add the chicken to a large mixing bowl, sprinkle the cornstarch out over it, and toss to evenly coat the chicken.
Add 3 tablespoons of olive oil to a large wok and heat it over medium heat.
Add the chicken to the hot oil, spreading out in a single layer- preferably without the thicken touching.
Cook the chicken for 5 minutes, per side, or until the outside's golden brown & the chicken's cooked all the way through. Transfer the chicken to a waiting plate, and cover it with tinfoil to keep it warm.
Add the remaining two tablespoons of oil to the wok. Add the green beans to the skillet, sauteing them for 2 minutes, stirring occasionally. Stir in the onions, and continue cooking the vegetables another 5 minutes, or until tender and just cooked through. Do not over cook them!
Return the chicken to the skillet, stirring to evenly combine with the veggies. Pour the sauce into the skillet, tossing everything to evenly coat.
Serve the chicken & green beans over top a bed of white rice, or on a plate with rice on the side.