Preheat the oven to 180°C.
Tap mussels and discard any that do not close.
Fry onions and garlic in oil for 6 minutes until softened.
Simmer wine, add mussels, cover, and cook for 5 minutes until opened.
Strain the mixture through a sieve, reserving 100ml of the cooking liquor.
Discard any mussels that haven't opened.
Sauté the remaining onion for 4 minutes until soft.
Add tomatoes, tomato purée, wine, and the reserved liquor.
Cook for 4 minutes and stir in the basil and mussels.
Transfer to a dish, top with parmesan, and bake for 5 minutes until golden.
Serve warm with crusty white bread.