Whisk together 1/4 cup water, 1/4 cup low-sodium soy sauce, 1/4 cup rice vinegar, 1/4 cup brown sugar, 1 tablespoon sesame oil, 2 teaspoons Sriracha, minced garlic, and 1 teaspoon ground ginger in a mixing bowl; set the teriyaki sauce aside.
Heat a large skillet or wok over medium-high heat and add 1 tablespoon oil.
Add the cubed chicken thighs to the hot pan and stir-fry until mostly cooked through and starting to brown, about 4–6 minutes.
Add the vegetables and cashews, then cook until vegetables are heated through and tender (about 3–4 minutes for frozen or 5–7 minutes for fresh) and the chicken reaches 165°F internally.
Pour the prepared teriyaki sauce over the chicken and vegetables and fold to coat; cook briefly until sauce is warmed through.
To thicken the sauce (optional), whisk 1 tablespoon cornstarch with 1 tablespoon water until smooth, push the stir-fry to one side, add the slurry to the pan, whisk into the sauce, then fold everything together and cook until the sauce reaches the desired thickness.
Transfer to a serving dish and garnish with sliced scallions and sesame seeds.
