Make pastry. Mix the flour and salt. Grate in the cold butter and stir to coat. Add the water a spoonful at a time until the mix clumps. Press into a flat block, wrap and chill for 30 minutes.
Cut salmon. Pat the salmon dry, cut it into 4 even pieces and remove any pin bones. Keep it in the fridge.
Wilt greens. Melt the butter in a large frying pan on a high heat. Add the greens and stir for about 1 minute, until collapsed.
Dry greens. Tip into a sieve and cool. Press with a wooden spoon, then with kitchen paper. Stir in the crème fraîche, lemon juice, nutmeg, salt and pepper.
Roll pastry. Divide the pastry into 4. Roll one piece very thinly, to about 21 cm by 15 cm, on a lightly floured surface.
Fill. Spread a quarter of the greens in the middle of the pastry. Put a piece of salmon on top and season lightly.
Wrap parcels. Dampen the corners with cold water. Fold in the short ends, dampen the long edges, then fold them over to enclose the salmon. Turn seam side down onto a lined tray. Repeat with the rest.
Chill. Chill the parcels for at least 20 minutes, covered with cling film if you are leaving them longer.
Heat oven. Heat the oven to 220°C (200°C fan, gas mark 7) with a shelf near the top.
Glaze. Slash the top of each parcel 3 times on the diagonal. Brush with beaten egg.
Bake. Bake on the high shelf for 20 minutes, until crisp and golden. The middle of the salmon should reach at least 63°C.
Rest and serve. Rest for about 5 minutes. Serve with foaming hollandaise and dill.
