Let the puff pastry thaw. Preheat the oven to 180 ° (convection).
Peel the onion and cut into thin slices. Peel the pears, quarter them and also cut into small slices.
Fry the onions in the butter, add the pears and fry for about 5 minutes. Deglaze with crème fraîche and season with salt and pepper.
Place the puff pastry slices on a baking sheet (or on a grid lined with baking paper), brush with the onion and pear mixture and bake for about 15 minutes.