Clean the mussels, put them in a pot with a lid and cook on slow fire until they open.
Remove from the heat, remove the mussels from their shells and set the filtered cooking water aside.
In a large skillet, saute the garlic in oil; add the chopped tomatoes and the basil leaves.
Season with salt and add the mussels with their cooking liquid. Sprinkle with chilli and cook for 5 minutes.
Serve hot, with croutons.