110g diced unsalted butter, softened, plus an extra knob for frying the mushrooms
75g baby chestnut mushrooms, roughly chopped
1 tbsp fresh flatleaf parsley, chopped, plus extra to garnish
1 tbsp fresh tarragon, chopped, plus extra to garnish
Salad or steamed veg to serve
Preparation
Heat the oven to 180°C. Put the potatoes in a pan of cold salted water and bring to the boil. Turn the heat down and simmer for 5-6 minutes until just starting to become tender. Drain well, then put in a roasting tin, drizzle with oil and season with salt and pepper. Roast for 20-25 minutes, tossing twice, until golden and crisp. Grate over the lemon zest and toss well, adding extra sea salt to taste.
Put the fish in a roasting tin or baking dish lined with non-stick baking paper and dot all over with the 25g butter. Pour a glug of vermouth/wine over the fish, then cover the tin/dish tightly with foil and bake for 18-20 minutes until the fish is just cooked through and feels firm to the touch. Spoon over the cooking juices and set aside to rest.
For the hollandaise, put the vinegar, shallot, peppercorns and bay leaf in a small heavy-based pan. Reduce to 1 tbsp by bubbling over a high heat, then strain immediately into a heatproof bowl (discard the solids). Set the bowl over a pan of barely simmering water (don’t let the bowl touch the water). Whisk in the egg yolks and a cube of butter, then whisk in the remaining butter, a cube at a time, until incorporated and thickened. Add the lemon juice and season to taste. Keep warm.
Meanwhile heat an extra knob of butter in a frying pan and fry the mushrooms until golden. Stir into the hollandaise along with the herbs.
Serve the fish with the hollandaise and the potatoes, plus some seasonal salad leaves or steamed vegetables.
