Remove the parsley leaves from the stalks. Put the parsley, anchovy fillets, capers and garlic on a chopping board and chop all together so that everything is cut up finely and starts to mix.
Transfer the mixture to a small bowl and add the mustard, lemon juice, a few grinds of pepper and the olive oil. Mix all together and refrigerate until ready to use (will keep a few hours well, a day or two not bad but not as bright colors).
Get a skillet/frying pan hot over a medium heat and add the butter.
Swirl the butter round a little so it covers all the pan then add the bass fillets, skin sound down.
Allow the fish to cook most of the way through - you will see it cook from the outside in.
When the fish is still a bit uncooked in the middle, carefully turn it over. Cook for a couple more minutes, turn back to skin side down for a minute then serve.
Stir the salsa and serve with the fish. Sides as you prefer but greens such as green beans and potatoes or salad work well.