Clean the mussels by discarding any with broken shells, remove the beards and using the back of a knife; scrape off any barnacles. Rinse the mussels in cold water to drain out any grit or bits of broken shell. Drain and keep the cleaned mussels refrigerated until use.
Heat light olive oil in a heavy bottomed, large saucepan.(avoid using non-stick pans as the mussels will scratch the cooking surface) Add the shallots, garlic, chillies, paprika and sauté over low heat until the shallots have softened. Turn up the heat, add white wine and cook for 2 minutes. Add tomatoes and sherry vinegar, cover and simmer for 15 minutes.
While the sauce is cooking, place walnuts, garlic, bread, parsley and salt in a food processor and pulse until well combined. Slowly add the oil while pulsing to get a pesto-like sauce.
Taste and season the sauce and add the mussels. Increase the heat, stir once then cover with a lid. Simmer for 3-5 minutes, or until the mussels have opened. Discard any mussels that have not opened. To serve, portion some mussels and sauce and top each portion with a few teaspoons of picada. It’s best eaten on its own orwith bread (for a more substantial meal, add some orzo or rice when cooking the sauce)