Heat oil in a large, heavy pot over medium heat, melt butter.
Add the garlic and onion, stirring occasionally until onion is soft and fragrant - about 3 minutes.
Turn heat up to high, add wine and bring to boil. Simmer 2 minutes until the harsh alcoholic smell is gone.
Stir in tomatoes, add mussels stir to coat in sauce a bit.
Cover, lower heat to medium high and cook, shaking pot once or twice.
The mussels are cooked when they are open - around 6 to 8 minutes.
Toss through parsley and season to taste with salt and pepper. Be careful with the salt as the mussels are already salty, so taste first.
Drizzle with extra virgin olive oil, and serve with lemon wedgeds on the side so people can use as much as they wish. Serve immediately with crusty bread to mop up the sauce!
