For the risotto, heat the oil in a large heavy-bottomed saucepan, add the onion and fry over a low heat until softened but not coloured.
Add the garlic and chilli and fry for one minute.
Add the rice and stir over a gentle heat until the rice grains turn translucent.
Add the wine, turn up the heat and boil until almost all of the liquid has disappeared. Add the vegetable stock, one ladleful at a time, and stir constantly until all the stock has been absorbed and the rice is tender.
Stir in the tomatoes, parsley and butter and season, to taste, with salt and freshly ground black pepper. Take off the heat, cover with a lid and leave for 3-4 minutes.
For the mussels, heat the olive oil in a large saucepan until very hot, add the mussels, cover with a lid and shake the pan over the heat until the mussel shells have opened. Discard any that are cracked or have not opened.
To serve, spoon the risotto into a serving bowl, top with the mussels and serve.