Prepare the ingredients before cooking.
Cut the chicken thighs into bite-sized pieces.
Heat the vegetable oil in a large skillet or wok.
Add the chicken to the skillet and sauté until browned.
Stir in the minced garlic and cook until fragrant, about 30 seconds.
Combine soy sauce, sugar, rice vinegar, and black pepper in a bowl and stir until the sugar dissolves.
Pour the sauce over the chicken and stir to coat.
In a small bowl, mix cornstarch with 2 tablespoons of water to create a slurry.
Add the cornstarch slurry to the skillet and simmer until the sauce thickens, about 1-2 minutes.
Taste and adjust seasoning if necessary before serving.
Serve hot, garnished with chopped green onions over rice or noodles.