Mix together soy sauce, chicken broth, sugar, vinegar, cornstarch, ginger, garlic and red pepper flakes.
Add in chicken and stir to coat. Allow to marinade for 15 minutes in refrigerator.
Heat 1 Tbsp sesame oil in a large pan over medium heat.
Add in carrots and broccoli and cook until tender, add in asparagus near the end.
Remove from pan and keep warm.
Add 1 Tbsp oil to pan and remove chicken from marinade and reserve liquid.
Add chicken to hot oil and cook until chicken is cooked through. Add in cooked vegetables and uncooked snap peas.
Add remaining marinade to skillet and stir until chicken and veggies are coated and marinade begins to thicken. Add in walnuts and stir to incorporate.
Serve over rice if desired.