Place the puff pastry sheets next to each other and let them thaw for about 5 minutes. Cut the plates in half crosswise. Whisk egg yolk and 1 tablespoon of water. Fold the corners of the pastry halves towards the middle and brush with egg yolk.
Peel and halve onion and cut into 12 slices. Put smoked salmon on the dough pieces and put one onion column on each. Spread Hollandaise sauce over it. Put the puff pastry particles on a baking tray lined with baking paper and bake in a preheated oven (electric range: 200 °C/ gas: level 3)
Bake for 12-15 minutes. Cut olives into slices and spread them on the pieces. Arrange the puff pastry particles on a plate and garnish with chervil. Makes 12 pieces