Cook the rice. Rinse the rice under cold water until the water runs mostly clear.
Add rice, water, and a pinch of salt to a pot or rice cooker. Cook according to package directions. Fluff and keep covered.
Prep the sauce. In a small bowl, whisk soy sauce, water, brown sugar, rice vinegar, mirin (if using), and sesame oil.
In another small cup, mix cornstarch with cold water to make a slurry. Set both aside.
Season the chicken. Pat the chicken dry. Sprinkle lightly with salt and pepper.
Dry chicken browns better and keeps the sauce from thinning out.
Sauté the veggies. Heat half the oil in a large skillet over medium-high heat. Add broccoli, carrots, and bell pepper. Cook 3–4 minutes until crisp-tender.
Remove to a plate.
Cook the chicken. Add remaining oil to the same skillet. Add chicken in an even layer. Let it sear for 2 minutes before stirring.
Cook 5–7 minutes total, until browned and cooked through. Add garlic and ginger in the last minute, stirring until fragrant.
Add the sauce. Pour the teriyaki sauce base into the skillet. Bring to a gentle simmer.
Stir in the cornstarch slurry and cook 1–2 minutes until the sauce thickens and turns glossy.
Combine. Return the veggies to the pan and toss to coat everything evenly in sauce. If the sauce is too thick, add a splash of water. If it’s thin, simmer another minute.
Assemble bowls. Spoon rice into bowls, top with teriyaki chicken and veggies, and garnish with green onions, sesame seeds, and a little heat if you like.