150g unsalted butter, cut into cubes and kept cold, plus 1 tbsp, softened
1 whole side of salmon (1kg)
15g dill, finely chopped
1 lemon, zested
Preheat the oven to 200°C, fan 180°C, gas 6. Cut thin slits into each potato with a sharp knife, stopping about two-thirds of the way down. Toss them with the fennel in a large mixing dish with the olive oil until completely coated. Place the fennel in a large roasting tin and roast for 30-35 minutes, stirring occasionally, until lightly caramelized and beginning to crisp.
Meanwhile, in a pan with 100ml water, combine the vinegar and shallot. Bring the water to a boil and then reduce by two-thirds (it will take 5-8 mins). Strain into a jug, reserving the shallot; set aside.
1 tbsp softened butter, rubbed all over the salmon flesh, seasoning well Remove the roasting tin from the oven, push the vegetables to one side, and place the salmon, skin-side down, diagonally across the tin in the empty space. Preheat oven to 350°F and roast for 25 minutes.
In the meantime, place 2 tablespoons of the vinegar reduction in a bowl set over a pan of gently simmering water over medium-low heat, making that the water does not touch the bowl. After the reduction has warmed up, whisk in a cube of cold butter until it has melted.
Repeat until the mixture thickens slightly and becomes glossy, adding 1 cube at a time and whisking constantly (about 4-5 mins). Then, 4-5 cubes at a time, add the butter. Taste and season with salt once all of the butter has been added; reduce even more if you want it to be sharper.
You can also add a little of the shallot that was set aside for texture. Remove the sauce from the heat and keep it warm until ready to serve, using a piece of clingfilm to prevent a skin from forming on the surface.
Serve the salmon with the potatoes and fennel, garnished with dill and lemon zest. Stir the beurre blanc and serve on the