In a large, deep skillet, bring 2 inches of water to a boil.
In a medium bowl, toss the chicken with 2 tbsp. teriyaki sauce. Marinate 10 minutes.
Salt the water. Add the green beans; cook until crisp- tender, about 3 minutes. Using a slotted spoon, transfer the green beans to a bowl of ice water; let cool. Drain; pat dry.
In a medium bowl, combine the remaining 2 tbsp. teriyaki sauce, the chicken stock, mirin, cornstarch and chile pepper to make the sauce.
In a large nonstick skillet or a wok, heat 1 tbsp. oil over high. When it ripples and begins to smoke, add the chicken in a single layer; stir-fry until browned, 2 to 3 minutes. Transfer to a plate. Add the remaining 1 tbsp. oil; cook the mushrooms until browned, about 3 minutes. Add the green beans; cook until they blister, 1 to 2 minutes. Add the scallions, garlic and ginger. Toss for 1 to 2 minutes, then add the sauce; toss again. Return the chicken to the skillet; toss to combine. Serve with cooked rice.