Cook the brown rice. Rinse 1 cup uncooked brown rice, then cook according to package directions. Fluff and set aside. If you have leftover rice, this is a great time to use it.
Prep the chicken. Pat the chicken dry and cut into bite-size pieces.
Season lightly with salt and pepper. This helps the chicken sear instead of steam.
Make the sauce base. In a small bowl, whisk together soy sauce, honey, rice vinegar, mirin (if using), water, and sesame oil. In another small bowl, mix cornstarch with water to make a slurry and set aside.
Sear the chicken. Heat the oil in a large skillet over medium-high heat.
Add the chicken in a single layer and let it cook undisturbed for 2–3 minutes to brown. Stir and cook another 3–4 minutes until nearly cooked through. Transfer to a plate.
Sauté aromatics. In the same pan, reduce heat to medium.
Add garlic and ginger and cook for 30 seconds, stirring to prevent burning. If using veggies, add them now and sauté 2–3 minutes until crisp-tender.
Add sauce and thicken. Pour the sauce base into the pan. Once it simmers, stir in the cornstarch slurry.
Cook 1–2 minutes, stirring, until the sauce turns glossy and slightly thick.
Return the chicken. Add the chicken and any juices back to the pan. Toss to coat and simmer 2–3 minutes until the chicken is fully cooked and the sauce clings nicely.
Finish and serve. Taste and adjust with a splash more soy sauce, honey, or vinegar if needed. Serve over warm brown rice.
Garnish with green onions and sesame seeds.