Sweet collard greens are an easy way to take this chicken stir-fry to another level, flavor-wise and health-wise. Slideshow: More Stir Fry Recipes
In a small bowl, whisk together the chicken stock, soy sauce, crushed red pepper, black pepper and cornstarch. Set aside.
In a large skillet, heat 2 tablespoons of the olive oil. Add the chicken and stir-fry over moderately high heat until no longer pink and cooked through, about 5 to 7 minutes. Transfer the chicken to a plate.
Heat the remaining 2 tablespoons of oil in the skillet. Add the onion and stir-fry for 3 minutes, then add the garlic and cook for 1 more minute.
Stir the chicken stock mixture into the vegetables and cook until thickened, about 1 to 2 minutes. Stir in the collard greens and cook until tender, about 3 minutes. Stir in the chicken and cook until heated through, about 1 to 2 more minutes. Season with additional soy sauce if desired. Serve with rice.