With the electric pressure cooker on the saute mode, heat until “hot.” Add the butter and onion. Cook until the onion is softened. Stir in the garlic and chicken stock. Place the Brussel sprouts on top. Place the lid on the pressure cooker and set the valve to sealing. Program the pressure cooker on “manual” or “pressure cook” for 5 minutes. When the cooking cycle has completed, perform a quick release. Remove the lid and stir in the remaining ingredients.