Add chicken, cornstarch, salt, and pepper to a large bowl and toss to combine.
In a separate medium bowl, add water, soy sauce, brown sugar, sesame oil, and garlic. Mix well and set aside.
To a large nonstick skillet over medium-high heat, add oil. Once hot, add chicken. Do not overcrowd the pan. Depending on the size of the skillet, you may have to work in batches. Cook for 2-3 minutes per side, or until browned and cooked through to an internal temperature of 165°F. Transfer the chicken to a plate, and tent to keep warm.
Add the onions and jalapenos to the skillet and cook until softened (about 4-6 minutes).1
Add the chicken and the soy sauce mixture to the pan and stir to combine. Continue to cook for 2-3 minutes.
Serve garnished with sesame seeds over cooked white rice.