Wash the asparagus. Peel the white asparagus completely, in the case of the green asparagus only the lower third. Cut off woody ends. Cook the asparagus spears in boiling water with a strong pinch of salt and sugar, 1 tablespoon butter and lemon juice for 5-8 minutes until al dente, depending on the thickness of the spears.
Remove the asparagus, rinse with cold water, drain and allow to cool.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Line baking tray with baking paper.
Cut the puff pastry into 4 equally sized pieces. Spread the ham slices with herb crème-fraîche. Place 1 slice of ham in the middle of each pastry sheet. Divide asparagus into 4 portions and place 1 portion on each slice of ham.
Whisk the egg yolk and milk and brush the edges of the pastry sheets. Carefully roll up, press the ends together and place the packs on the baking tray.
Brush the outside of the packet with the remaining egg yolk milk and bake in a hot oven on the middle shelf for 30-35 minutes until golden brown. Remove and enjoy warm.