Preheat the oven to 180C/350F/Gas 4.
For the mussels, tap the mussels against a hard surface and discard any that do not close.
Heat the oil in a pan and gently fry the onions and garlic for 5-6 minutes, or until softened. Pour in the white wine and bring to a simmer, then add the mussels and cover with a lid for 4-5 minutes, or until the mussels have opened. Strain through a sieve, reserving the cooking liquor and discarding any unopened mussels.
For the passata sauce, heat the oil in a pan and gently fry the onions for 3-4 minutes, or until softened. Add the tomatoes, tomato purée and wine. Measure out 100ml/3½fl oz of the mussel cooking liquor and add to the passata. Cook for a further 3-4 minutes, then stir in the basil and cooked mussels.
Spoon the mussels into a gratin dish, sprinkle over the parmesan and bake for 4-5 minutes, or until golden-brown and bubbling on top. Serve with crusty white bread.