To clean the mussels, put them in a large bowl with 2 quarts of water and the flour and soak for 30 minutes, or until the mussels disgorge any sand. Drain the mussels, then remove the "beard" from each with your fingers. If they're dirty, scrub the mussels with a brush under running water. Discard any mussels whose shells aren't tightly shut.
In a large non-aluminum stockpot, heat the butter and olive oil over medium heat. Add the shallots and cook for 5 minutes; then add the garlic and cook for 3 more minutes, or until the shallots are translucent. Add the tomatoes, saffron, parsley, thyme, wine, salt, and pepper. Bring to a boil.
Add the mussels, stir well, then cover the pot, and cook over medium heat for 8 to 10 minutes, until all the mussels are opened (discard any that do not open). With the lid on, shake the pot once or twice to be sure the mussels don't burn on the bottom. Pour the mussels and the sauce into a large bowl and serve hot.
