Slice and toast your bread by brushing it with olive oil or butter and broiling it for 2-3 minutes or until golden brown. Set aside.
You'll want to use either a pan that is 4 to 5 inches deep or a 4 to 5 quart pot for this, so you have room to toss the mussels around as they cook.
Heat the olive oil over medium-high heat and sauté the onion for 4 to 5 minutes or until soft.
Add the garlic and cook for 1 to 2 minutes or until fragrant.
Deglaze the pan with the white wine and cook for one minute.
Add the broth, tomatoes, salt and pepper and bring to a boil.
Add the butter and allow it to melt into the sauce.
Add the mussels, toss to coat in the sauce and cover. Boil for 3 minutes, stir them around, cover again and cook for 3 more minutes.
Remove the pan from the heat, add the parsley and toss to combine.
To serve the mussels, pour them into a large, shallow serving dish alongside some crusty bread for dunking in the sauce.
Garnish with additional parsley before serving (optional)