Cut chicken thighs into 1-inch pieces and pat dry with paper towels. In a bowl, toss the chicken with cornstarch, salt, and pepper until evenly coated.

Heat vegetable oil in a large skillet or wok over medium-high heat. Add chicken in a single layer and cook for 3-4 minutes per side until golden brown and cooked through. Remove chicken from skillet and set aside.

In a medium bowl, whisk together orange juice, soy sauce, rice vinegar, brown sugar, grated ginger, minced garlic, and red pepper flakes. In a small bowl, dissolve cornstarch in water to create a slurry.

Pour the orange sauce mixture into the same skillet and bring to a simmer over medium heat. Stir in the cornstarch slurry and cook for 1-2 minutes until the sauce thickens.

Return the cooked chicken to the skillet, tossing to coat evenly with the orange sauce. Cook for another 1-2 minutes, allowing the chicken to warm through and absorb the sauce.

Garnish with sesame seeds and sliced green onions, then serve immediately with rice and steamed vegetables.