First prepare the sauce: In a small pot, add the water, soy sauce, honey, rice vinegar, garlic, ginger, and red pepper flakes. Heat over medium high, stirring until smoothly combined. Bring to a boil. Add the cornstarch slurry. Whisk constantly until the sauce has thickened, about 1 to 2 minutes or until sauce has thickened.
In a large, deep sauté pan or wok, heat 1/2 tablespoon oil over medium-high heat. Once the oil is hot but not smoking, add the chicken and a few dashes of soy sauce. Cook until the chicken is golden on all sides and fully cooked through, about 4 to 6 minutes. With a large spoon, remove to a plate and set aside.
Heat the remaining 1/2 tablespoon oil. Add the bell pepper and asparagus. Cook for 3 to 4 minutes, or until softened and lightly browned.
When the vegetables are ready, add the water chestnuts and return the chicken to the pan. Pour in the sauce and toss to coat. Let simmer 1 to 2 minutes to warm the chicken through. Stir in about half of the green onions. Sprinkle the sesame seeds and remaining green onions over the top. Serve warm, with additional red pepper flakes and/or soy sauce as desired.
