In a bowl, toss the chopped chicken thighs with cornstarch, salt, and black pepper until evenly coated.
Heat the neutral oil in a large skillet or wok over medium-high heat. Add the chicken in batches if necessary, and cook until golden brown and crispy on the outside and cooked through on the inside, about 6-8 minutes. Remove the chicken from the pan and set aside.
In the same pan, add the sliced onion and jalapeños. Sauté for 2-3 minutes until the vegetables are slightly softened but still vibrant. Remove the vegetables from the pan and set aside.
In a bowl, whisk together the brown sugar, soy sauce, rice vinegar, sesame oil, water, and cornstarch until combined.
Return the skillet to medium heat and pour in the sauce. Cook for a few minutes, stirring continuously, until the sauce thickens.
Add the cooked chicken and vegetables back into the skillet, tossing to coat everything evenly in the sauce. Cook for an additional 1-2 minutes to heat through.
Remove from heat and stir in the chopped cilantro. Garnish with green onions and sesame seeds. Serve hot with rice or your favorite side.
