Clean the mussels and discard any that don’t close when sharply tapped.
Place the mussels in a large saucepan and cover with the wine. Cook for 3–4 minutes, shaking the pan once or twice, until the mussels are open. Transfer them to a bowl and discard any that do not open. Strain the cooking liquid into a separate bowl to remove any grit or impurities and set this and the mussels aside.
Melt half the butter in a pan over a low heat and add the shallots and garlic.
Add the bacon, increase the heat to medium and cook for a few minutes until the bacon is crisp.
Now add the tomatoes and cook until softened.
Mash the flour and the remaining butter in a bowl, then add this mixture to the pan, piece by piece. At the same time, slowly add the mussel cooking liquid. If you find that the sauce is too thick, add a little water to loosen the mixture.
Reheat the mussels in the sauce, then sprinkle with the parsley. Serve immediately with some crusty bread.