Cut the asparagus into 1-inch pieces. Set aside. Cut the chicken into bite-sized pieces. Set aside.
Heat oil in a large rimmed skillet over medium-high heat.
Add chicken to the skillet and cook till no longer pink.
While the chicken is cooking, make the sauce. Whisk together soy sauce, vinegar, hoisin sauce, ginger, garlic, chili flakes, sugar, and cornstarch.
Add the asparagus to the skillet with the chicken. Pour the sauce over the chicken and asparagus. Stir to coat.
Simmer until the asparagus is tender and the sauce has thickened.
Plate with rice and sprinkle green onion on top.