Marinate the Chicken: Cut boneless chicken thighs into bite-sized pieces, place in a bowl, season with black pepper and salt, and marinate for at least 15 minutes.

Heat the Skillet: Heat vegetable oil in a skillet over medium-high heat until it shimmers, add marinated chicken in a single layer, stir-fry for 5-7 minutes until golden brown, then remove and set aside.

Sauté the Vegetables: In the same skillet, add more oil if necessary, then sauté sliced onions and green bell peppers for 2-3 minutes until softened.

Add Garlic: Quickly add minced garlic to the skillet, stir-fry for about 1 minute until fragrant, and be careful not to burn it.

Combine Chicken and Sauces: Return browned chicken to the skillet, add soy sauce, oyster sauce, and sugar, stir well for 2-3 minutes.

Thicken the Sauce: Create a cornstarch slurry, add to skillet, and stir until sauce thickens, about 1-2 minutes.

Final Cooking: Adjust seasoning if needed, cook for another 5 minutes until chicken is cooked through.

Garnish and Serve: Remove from heat, garnish with chopped green onions, and serve hot over rice or noodles.