Cut the chicken thighs into 1 ½-2 inch pieces. Toss in a mixing bowl with the salt, pepper, ginger, and sesame seeds.
Heat the olive oil in a wide pan over high heat. Cook the chicken in the hot oil until all sides are a charred brown. While the insides may still be raw, push the chicken to the outer edge of the pan.
Reduce the heat to medium and toss in thin-sliced bell pepper and chopped mushrooms with the second amount of olive oil. Occasionally stir the veggies as they cook down to tender. Toss the ingredients together.
Push the chicken and veggies back to the outer edge of the pan and reduce the heat to low. Set in the center of the pan all the remaining ingredients. Wait for the butter to melt and the garlic, ginger, and sesame seeds become fragrant – about 45 seconds.
Stir the Kung Pao sauce together and toss all the ingredients together once more until everything is coated in sauce.
Serve hot with chopped fresh green onion.