Begin by slicing the boneless chicken thighs into bite-sized pieces. In a mixing bowl, season the chicken with salt, pepper, and dark soy sauce, ensuring each piece is evenly coated. Add a slurry of cornstarch mixed with a splash of water and drizzle a tablespoon of oil. Cover and let marinate for 10 minutes.

In a medium saucepan, heat two tablespoons of vegetable oil over medium heat. Once shimmering, add minced garlic and chopped onions, sautéing for 2-3 minutes until soft and fragrant. Sprinkle in black pepper, stir in butter, and pour in water or stock. Let simmer for 1-2 minutes.

Create a cornstarch slurry by mixing equal parts cornstarch and water. Gradually add this to the simmering sauce, stirring until thick and glossy, about 30-60 seconds. Remove from heat and set aside.

Heat a wok or large skillet over high heat and add a tablespoon of oil. When hot, add the marinated chicken, spreading it out for even cooking. Stir-fry for 4-5 minutes until golden brown. Push to the sides of the wok.

In the center of the wok, add sliced onions and bell peppers. Stir-fry for about 2 minutes until softened but still vibrant. Incorporate the cooked chicken with the veggies.

Pour the thickened black pepper sauce over the stir-fried chicken and vegetables. Stir continuously for 1-2 minutes to coat evenly, then remove from heat.

Serve immediately over steamed rice or tossed with noodles, garnished with fresh black pepper or sesame seeds.