In a large stockpot, add the oil, butter and onions. Cook onions until soft and translucent. Stir occasionally. Then add the garlic and cook for a minute.

Add the mussels along with white wine then cover with a lid. Let the wine cook out and mussels steam. Once the mussels have opened up, add the parsley, cover and let it cook for another minute. Season with a sprinkle with salt or pepper (optional). Shake the stockpot and keep covered until mussels have opened up.

Remove from heat and transfer mussels along with the sauce into one large serving bowl or individual serving bowls.

Eat immediately while hot and serve with sliced baguette bread to dip in the liquor.