Grease a baking tray with a little of the butter, then place the fish on top, dark skin-side up. Place the lemon slices and herbs in the cavity, then pour over the wine, dot with the remaining butter and season. Loosely cover with a sheet of greaseproof or baking paper and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until just cooked through.
Meanwhile, to make the sauce, place the shallot, vinegar and wine in a saucepan and boil until reduced to 2 tablespoons. Whisking constantly, add the butter, a cube at a time, until the sauce thickens and turns creamy. Season, add a little extra butter if too sharp and stir through the tarragon.
Serve the tarragon sauce alongside the baked turbot.