Wash the lemon hot, cut off 6 thin slices and set aside. Squeeze the rest of the lemon. Peel onions and chop finely. Clean the mushrooms, wash if necessary. Put a good 100 g of mushrooms aside, chop the rest of the mushrooms.
Heat 3 tablespoons of oil in a large pot. Brown the chopped mushrooms in it. Season with salt and pepper. Add onions and fry briefly. Sprinkle with 1 tsp. paprika, sauté briefly. Deglaze with 800 ml water and lemon juice. Stir in broth and cream, up to 3 tbsp, bring to the boil. Simmer covered for about 20 minutes.
Defrost puff pastry slices side by side for about 10 minutes. Preheat the oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Line a tray with baking paper. Depending on the size, cut out 18-24 stars and place them on the tray. Whisk the egg yolk and the rest of the cream, brush the stars with it. Bake in a hot oven for about 10 minutes.
Wash the herbs and shake dry. Pluck off dill flags, chop parsley roughly. Cut the remaining mushrooms into slices and fry them in 1 tbsp. hot oil. Add parsley to the soup, puree finely and season to taste. Serve soup with mushrooms and lemon slices. Garnish with sour cream, puff pastry stars and dill.