Fry the onions in the olive oil over low heat for five minutes. You want the onions to soften, not caramelise. While the onions are frying, cut the chorizo. Dice half of it into small blocks, and cut the other half in thicker slices. Add the chorizo, garlic and tomato paste to the onion and fry for a few more minutes. Add the sherry and cook for a minute. Next add the tomato and the saffron along with the liquid is soaked in. (The saffron lends a wonderful extra depth to this dish, but as it’s a bit pricey you could leave it out if you don’t want to spring for it. You do still need to add the ½ cup of water though.) Cover with a lid and cook over medium heat for 30 minutes. You want the sauce to be quite dry, so have a look after 15 minutes and if it is too watery, lift the lid.
In the meantime clean the mussels by pulling off any visible beard (sometimes a firm tug is required, the little critters cling to their beards) and knock of any obvious barnacles. Place in a colander and rinse just to make sure any sandy nastiness is removed.
Taste the tomato sauce for salt and adjust. (It will need at least half a teaspoon salt, possibly more depending on the salt levels of the chorizo you used. Remember, salt is flavour!) Tumble the mussels into the prepared tomato and chorizo sauce, cover and cook over high heat until the mussels open. Mussels cook incredibly quickly, so this takes only about two minutes. Discard any mussels that did not open, scatter over the parsley and serve right away.