Allow the butter to soften in a bowl. Rinse the lime in hot water, pat dry. Finely chop the chervil.
Place the butter and chervil in a bowl. Grate the lime zest over the top, season, mix well.
Using a peeler, peel the asparagus into thin strips, starting from just below the asparagus tip right down to the end, season with salt and pepper.
Cover and leave to infuse for approx. 15 mins, drain well. Halve the asparagus tips lengthwise.
On a lightly floured surface, roll out the pastry into a rectangle (approx. 20 x 30 cm). Cut into six 10cm squares and place on a tray lined with baking paper. Thin the egg yolk with a little water. Coat the edges of the dough with the egg & water.
Top the puff pastry
Fold the smoked ham slices and place in the middle of the pastry squares. Shape the asparagus strips into 6 little nests and place on top of the ham with the asparagus tips.
Slice the fish into pieces approx. 4 cm wide, season with salt. Place one piece of fish on top of each asparagus nest, top with the chervil butter. Bake for approx. 20 mins. in the lower half of an oven preheated to 200°C.