Defrost puff pastry slices at room temperature. Roll out dough sheets on a floured surface to squares (approx. 20x20 cm). Place in 4 small moulds (each approx. 15 cm Ø).
Leave corners over the edge. Sort the spinach and wash thoroughly. Peel onion and garlic. Cut garlic into slices and onion into cubes. Clean, wash and halve the mushrooms.
Melt 30 g fat in a large pot. Brown the onion and garlic. Fry the mushrooms. Add dripping wet spinach and steam for about 5 minutes until it has collapsed. Season with salt and pepper.
Heat the remaining fat and sweat the flour in it. Add milk while stirring and bring to the boil. Grate cheese and melt in the sauce. Season to taste with a little salt and pepper. Mix the sauce and spinach vegetables and fill into the moulds.
Whisk the egg yolk and a little water and brush the corners of the puff pastry with it, but not the edges. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 20-25 minutes. Turn salami slices into rosettes and serve the tartlets garnished with one slice each.