This is a great recipe to make during the spring, when asparagus is in season. However, feel free to switch out the asparagus for another vegetable during other times of the year.
Combine the tamari and rice vinegar in a small bowl, and set aside.
Season the chicken with salt and pepper. Heat oil in a large heavy skillet over high heat. Add garlic and ginger and stir until fragrant, about 30 seconds.
Add chicken to skillet and stir-fry until white, about 2 minutes. Add asparagus, bell pepper, onion and crushed red pepper, if desired, and stir-fry until vegetables are crisp-tender and chicken is just cooked through, about 3 minutes.
Add soy sauce mixture, stir, and cook for about 1 minute more. Serve immediately.