Prep your chicken and veggies: Slice the chicken into strips. Slice the onion into half moon shapes. Cut green beans into 2 inch pieces.
Combine the marinade ingredients together in a large bowl. Place the chicken into the marinade. Cover and refrigerate at least 30 minutes (up to overnight).
Make the sauce: Whisk together the chicken stock, soy sauce, cornstarch, rice vinegar and hoisin sauce. Set aside.
Heat a large skillet over medium heat. Cook the chicken until golden brown on both sides, about 4-5 minutes per side. Remove from the pan onto a separate plate.
Wipe the pan clean. Add sesame oil, garlic, ginger and onions. Cook until the onions become soft and translucent, about 5 minutes. Add the string beans to the pan and cook 3 more minutes.
Add the chicken back into the pan. Pour the sauce mixture into the pan and toss to coat. Simmer until the sauce thickens. Serve over cooked rice, if desired.