Preheat the oven to 210C/425F/Gas7. Line a baking tray with silicone or baking parchment.
For the salmon coulibiac, roll the pastry out to a thickness of 5mm and cut into two pieces about 40 x 25cm /16 x 10in. Place one piece onto the lined baking tray.
Cut off 2/3 of the whole fillet (roughly 800g), place on top of the pastry on the tray in the centre then cover with half the blanched spinach.
Roughly chop the remaining salmon (about 400g) and place into a food processor with the double cream and half the chives. Blend to a purée.
Season with salt and plenty of freshly ground black pepper, then spread over the top of the spinach to cover completely.
Finish with a scattering of chives and the rest of the spinach. Brush the pastry around the fish with a little of the beaten egg.
Lay the second piece of pastry over the top, taking care not to stretch it and press down firmly around the edge of the salmon.
Trim the edges with a sharp knife so there is only a little overlap around the edge. Brush the coulibiac all over with egg and season with salt and black pepper.
Place onto a baking tray in the oven and bake for 40-45 minutes until the salmon is cooked through and the pastry golden-brown.
For the roasted vegetables, place all the ingredients onto a roasting tray tossing the vegetables to coat with the olive oil. Season with salt and black pepper. Roast for 25-35 minutes until tender.
For the beurre blanc, bring the finely sliced shallots, white wine vinegar and white wine to the boil in a saucepan. Reduce the heat until the mixture is simmering, then continue to simmer until only two tablespoons of liquid are left in the pan.
Add one tablespoon of cold water and continue to simmer until only one tablespoon of liquid remains.
Reduce the heat to low, then gradually whisk in the butter, 25g/1oz at a time, waiting until all of the butter has melted and incorporated into the mixture before adding more.
Remove the mixture from the heat and strain it into a clean saucepan. Season, to taste, with salt and freshly ground black pepper, then stir in the chives. Set aside at room temperature until needed (do not refrigerate as the sauce will separate).
To serve, carve the salmon into thick slices and place onto a plate.
Pile the vegetables alongside and finish with a drizzle of beurre blanc.