Wash the mussels under running water, removing all of the beards and throwing out any broken or damaged mussels.
Add the mussels and stir well. Pour in the white wine and cover the container. Cook for about 10 min. until the mussels open. Stir from time to time, so that all of the mussels can heat up evenly.
Add the remaining 2 tbsp butter and sprinkle with olive oil. Season with salt and black pepper. Sprinkle with parsley and serve immediately.