Peel the onion and chop finely. Fry in a little oil in a saucepan and steam. Clean the mushrooms, cut into fine slices, add to the onions and cook with them. Then stir in the crème fraîche, add the fresh garlic with the garlic press and season with salt, pepper and sugar.
Preheat the oven to 220 ° C top / bottom heat.
Roll out the puff pastry on a baking sheet on parchment paper and cut into six equal rectangles (as square as possible). Place 1 - 2 tablespoons of the hot mushroom mixture in the middle of the puff pastry, then fold into a triangle and press the edges firmly together. Brush the top with condensed milk.
Bake in the middle of the oven for about 15 minutes until the puff pastry is golden brown. Serve hot!