In a medium bowl, combine 1/4 cup low sodium soy sauce, 2 tbsp mirin, 1 tbsp rice vinegar, 1 tbsp packed brown sugar, 1 tsp grated fresh ginger, 1 clove minced garlic, and 1 tsp sesame oil. Add 1 tbsp cornstarch and whisk until smooth. Add the chicken pieces, toss to coat evenly, and let marinate for at least 15 minutes at room temperature, or up to 30 minutes in the refrigerator for deeper flavor.

While the chicken marinates, chop all the vegetables as directed: broccoli into small florets, carrots julienned or thinly sliced, red bell pepper thinly sliced, and yellow onion thinly sliced.

In a small bowl, whisk together the additional 1/4 cup low sodium soy sauce, 2 tbsp mirin, 1 tbsp rice vinegar, 2 tbsp packed brown sugar, 1 tsp grated fresh ginger, and 1 clove minced garlic. Set this stir-fry sauce aside. In a separate small bowl, prepare the cornstarch slurry by whisking 1 tbsp cornstarch with 2 tbsp cold water until smooth.

Heat 1 tbsp olive oil in a large skillet or wok over medium-high heat until shimmering. Once hot, add the marinated chicken in a single layer (you may need to do this in batches to avoid overcrowding, which steams the chicken instead of searing it). Cook for 3-4 minutes per side, until browned and cooked through. Remove chicken from the skillet and set aside.

Add a little more oil to the skillet if needed. Add the broccoli, carrots, and yellow onion. Stir-fry for 3-4 minutes until they begin to soften and are slightly tender-crisp.

Add the red bell pepper and snap peas to the skillet. Stir-fry for another 2 minutes. Give the prepared teriyaki stir-fry sauce (from step 3) a quick whisk and pour it into the skillet. Bring the sauce to a gentle simmer.

Pour the cornstarch slurry into the simmering sauce, whisking continuously, until the sauce thickens to your desired consistency. It should coat the back of a spoon.

Return the cooked chicken to the skillet, tossing gently to coat everything evenly with the thickened sauce.

Divide the chicken teriyaki stir-fry evenly among 4 meal prep containers. If desired, portion out 1 cup of cooked brown rice or quinoa into each container alongside the stir-fry.

Let the stir-fry cool completely before sealing the containers and refrigerating. Garnish with sesame seeds and chopped green onions if serving immediately.