To make the sauce, heat a little olive oil in a pan and cook the shallots for about 5 mins until golden and soft.
Add the mushrooms and continue to cook for 10 mins, stirring occasionally.
Add the vinegar and let it come to the boil for 3 mins, then add the stock, season lightly and simmer for 30 mins, skimming at regular intervals.
Puree the mixture into a thick puree.
Make a sauce by passing some of the puree through a fine sieve into a clean pan, bring back to the boil and whisk in the butter to thicken and gloss the sauce.
Heat a non-stick frying pan until smoking hot, add a few drops of oil, then add the fish, skin side down.
Season with salt and pepper and press the fish down with a palette knife if it begins to curl up.
Once the skin is well browned, turn the fillets over and cook the other side; the whole process should take no more than 5-6 mins, depending on the thickness of the fish.
To serve
Pour the sauce around the purée, place the fish on top and add a few parsnip crisps for garnish.
