Bring the spaghetti to the boil.
Finely chop the onion and garlic and fry on a low heat in a glug of oil. They should become soft, but not brown if you can manage it. Add in the anchovy fillet, chopped. Cook for a few minutes – the anchovy will start to dissolve.
Cut all of the tomatoes in half and squeeze their juice into the pan, then add the shells to the pan too.
Cook for about 5 minutes until the oil/ tomato/anchovy combo makes a sauce. Add a bit more oil if it’s looking dry. Throw in the chilli and basil then season with salt and pepper if you want (I don’t).
Empty the mussels into the pan and stir them around. (If you’re using microwave ones, don’t cook them first!) Cover them with a lid and cook for 3 minutes. Uncover and remove any mussels which haven’t opened.
Keep the pan off the heat until the pasta is done, then drain the pasta, saving a little bit of the water. Pour the mussels and sauce into the pasta, add olives and toss evenly.
Heat with a lid on for another minute or two, until everything is hot, and serve with basil garnish and bread with oil and balsamic vinegar.