Unwrap ramen noodles and discard seasoning packet. In a medium pot set to high heat, boil 6 cups water. Stir in ramen noodles. Cook noodles for about 2 minutes, until still chewy. They will continue cooking in the sauce once combined. Drain noodles and rinse with cold water; set aside.
Drizzle oil in a large skillet to medium-high heat. Add chopped chicken and vegetables. Sauté for 7-8 minutes, until chicken is fully cooked and veggies are tender. Remove from heat. Add ramen to skillet.
In a small bowl, mix cornstarch and 2 Tbsp. water; set aside.
In a small pot, whisk all sauce ingredients together. Bring to a boil, then reduce heat to low. Stir in cornstarch-water mixture. Simmer and stir until thickened, 2-3 minutes. Mixture should coat the back of a spoon.
Pour sauce over noodle mixture. Mix to coat everything evenly in the sauce.
Set skillet to medium-high heat and cook mixture, until noodles are tender, about 3 minutes.
Garnish with sesame seeds and chopped scallions, if desired.