Heat oven to 200C/180C fan/gas 6 and line a large baking sheet with baking parchment. Unroll the pastry sheet and score a border about 1cm from the edge. Prick the middle area with a fork all over and glaze the border with the egg yolk. Bake the pastry for 15 mins until puffed up and almost cooked.

Meanwhile, put the salmon fillets in a microwaveable dish and cover with cling film. Cook in the microwave on High for 2 mins until just cooked. Set aside to cool for 1-2 mins, then flake into chunky pieces. Mix the crème fraîche with the mustard in a small bowl and season.

Take the pastry out of the oven and flatten the middle area with the back of a spoon. Cover the base with the crème fraîche sauce, then scatter the spinach and salmon flakes over. Bake in the oven for a further 5 mins or until the pastry is cooked and golden. Serve with dressed mixed leaves.