Finely chop the onion and cut the cleaned mushrooms into thin slices. Sauté onion and mushrooms in butter, season with salt, pepper and juice of one lemon. Dust with cornflour and cool. Cut the puff pastry into 2 halves. Cover one half with ham, then spread the onion and mushroom filling evenly on top. Place the second half of the dough on top and press the edges together.
Beat the egg and brush the dough with it. Decorate the surface of the strudel with a kitchen knife and bake in the oven at 200 °C for about 20 minutes. Finely dice the garlic and mix with the sour cream. Season strongly with salt and freshly ground pepper, then stir through with parsley and chives. Arrange ham and mushroom strudel with garlic sauce on plates, garnish with cherry tomatoes and parsley.
Our tip: Fresh chives are much more aromatic than dried ones!