Peel, wash and chop the potatoes. Cover and cook in salted water for about 10 minutes and drain.
Wash the asparagus. Peel white asparagus. Cut the woody ends off the whole asparagus. Cut the stalks diagonally into pieces. Bring 1.2 l water and 1 tsp. salt and sugar each to the boil. Cook the white asparagus for 7-8 minutes.
Cook the green asparagus for about 4 minutes. Lift out, rinse and drain. Put asparagus water aside.
Clean the mushrooms, possibly wash and halve them. Peel and chop the onion. Dab meat dry and cut into strips. Heat oil in a large pan. Brown the meat well all around.
Season with salt and pepper and remove. Fry the mushrooms in hot frying fat for about 5 minutes. Fry the onions briefly. Take out everything.
Let the puff pastry rest for about 10 minutes at room temperature. Heat the butter in the frying fat. Fry 4 tbsp. flour in it. Gradually deglaze with approx. 1 l asparagus stock and cream, bring to the boil and simmer for approx. 5 minutes.
Add potatoes, asparagus, meat and mushroom mix. Season with salt and pepper. Pour the ragout into an ovenproof dish (approx. 2.5 l capacity).
Unroll the dough, remove the baking parchment. Place doughs on top of each other and press them together slightly. Cut off a strip about 2 cm wide on the short side. Roll out dough sheet on a little flour 2-3 cm larger than the casserole dish.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Place the dough sheet over the mould and press it all around the edge of the mould. Whisk the egg yolk and 3 tbsp. water. Spread the dough with it. Cut the dough strips in half, form 2 asparagus spears and place them crosswise in the middle of the dough sheet.
Brush with egg yolk. Carefully punch a few holes into the dough with a punched nozzle (approx. 1 cm Ø) to allow the steam to escape. Bake in a hot oven for about 20 minutes.