For the spinach and tomato pouches, first chop the onion and garlic clove into small cubes. Sauté in butter and let cool. Wash and chop the spinach.
Blanch briefly and let cool well, squeezing out the excess water. Cut the tomatoes and mozzarella into small cubes.
Mix with the spinach, garlic, onion cubes and the crème fraiche and season well with salt and pepper. Cut the cool puff pastry lengthwise into two strips of equal width and then again into 4 squares each (8 pieces of pastry in total).
Cut three to four slits in the center of the top 4 squares with a knife. Now place the spinach-tomato filling on the lower pieces of dough and place the upper cut piece of dough on top of each. Press the edges well with a fork.
If necessary, brush with egg yolk and bake in the oven at 180 degrees for about 30 minutes until golden brown.