Preheat the oven to 425°F (220° C).
Cut the asparagus into 1/2" (1.25 cm) pieces and steam only until fork tender.
Drain and place in a mixing bowl; sprinkle with salt.
Set aside.
To make the sauce, melt the butter in a large saucepan and, when it is bubbly, stir in the flour.
Continue stirring until you have a smooth roux.
Add the milk and continue to stir and heat until the mixture begins to thicken into a smooth sauce.
Remove from the heat and whip in the cream and lemon juice.
Gradually whip in the beaten egg yolks and cheese and return the mixture to the heat while you stir for another minute or two.
Remove from the heat and add the chopped dill weed and slivered almonds.
Add the chopped, cooked asparagus and gently mix.
Roll out the defrosted puff pastry on a floured board to a large rectangle approx. 9" x 13" (23 x 33 cm).
Cut into squares approx. 3" x 4" (8 x 10 cm).
Place a spoonful of the asparagus filling in the centre of each square.
Moisten all four edges with the egg wash (1 egg beaten with a little water).
Fold over one corner of the pastry and seal the edges to make a triangular tart.
Make sure that all edges are firmly pressed together so that the egg wash will seal.
Place the triangles on an ungreased cookie sheet and brush each with more egg wash.
Bake for approx. 15-20 minutes.
Serve hot.