Thaw frozen puff pastry. Preheat the oven to 200 ° C top / bottom heat and line a baking sheet with baking paper.
Roll out the puff pastry on a floured work surface a little larger than the baking sheet. Place the dough on the baking sheet and use the knife to score an edge of about 2 cm and then pull it up.
Peel the onion, cut in half lengthways and cut into thin rings. Peel the garlic and press into the crème fraîche. Season to taste with salt, pepper and 2 - 3 teaspoons of lemon juice and spread on the dough. Cut the soft cheese into thin slices and place on top. Spread the onion rings on the tart. Wash the rosemary, shake dry, pluck off the needles and also place on the tart. Brush the edge of the pastry with 2 - 3 tablespoons of milk and bake in the oven for 30 - 40 minutes until golden brown.