Set a large pot or casserole over medium heat, pour the oil and add the butter and let it heat up for 1’. Add the onions and cook for about 5’ to soften. Add the garlic and cook for only 30’’.
Add the tomatoes and the peppers and cook for 2’ while tossing frequently. Deglaze with the wine, season with the salt and pepper and add the parsley.
Immediately add the mussels, turn up the heat and cook for about 5’ minutes until the mussels open.