Roll out the puff pastry on a baking sheet. Peel the pear and apple and slice thinly on a vegetable slicer up to the core. Mix the slices with the lemon juice and spread on the dough.
Preheat the oven to 200 degrees. Mix the sugar and cinnamon and sprinkle on the apple and pear slices. Melt the butter and drizzle on it. Bake in the lower third of the oven for 20-25 minutes until golden brown. Immediately cut the tart into pieces and serve with crème fraîche.