Preheat the oven to Gas Mark 6, 200°C (400°F).
Place the butter in a small bowl and beat in the tarragon with the chives, garlic and a little pepper and salt to taste. Spoon on to a sheet of clingfilm or non-stick parchment paper and shape into a roll about 2.5cm thick, then wrap thightly. Chill in the freezer for at least 10 minutes to firm up (or keep in the fridge for up to 48 hours until required, if time allows),
Cut the pastry into 8 even-sized sections and roll each one out on a lightly floured surface to a 23cm x 15cm rectangle, trimming down the edges as necessary. Place a salmon fillet in the centre of 4 of the pastry rectangles. Unwrap the tarragon butter, cut into slices and arrange on top, then cover with the spinach leaves. Season the spinach and add a little nutmeg.
Brush the edges of the pastry bases with a little of the beaten egg and lay a second sheet of pastry on top, pressing down to seal. Crimp the edges by gently pressing the edge of the pastry with the forefinger of one hand and between the first two fingers of the other hand. Continue all the way around the edge of the parcel, then repeat until you have 4 parcels in total. Using a sharp knife, make light slashes across each parcel but take care not to cut right ghrough.
Place a baking sheet in the preheated oven for a few minutes. Meanwhile, brush the pastry parcels with the remaining beaten egg. transfer to the heated baking sheet and bake for 25-30 minutes or until the pastry is cooked through and golden brown. Arrange the salmon parcels on warmed serving plates.