Prick the rectangles of puff pastry with a fork so that they do not puff up while baking.

Place them on a baking tray covered with baking paper and cover them with a silicone mat (or another sheet of baking paper and a heavy baking tray).  This will prevent them from puffing up too much.

Bake them in a preheated oven at 200°C – 400°F for 15 – 20 minutes.

Remove from the oven and leave the puff pastry rectangles to cool down on a wire rack.

Once the pastry has cooled, you can assemble your mille-feuille.

Spread some cream cheese on 4 puffy pastry rectangles and top them with some smoked salmon, capers, onion slices and chives (if you are using any). Top them with another 4 rectangles. Press down lightly with your hands to ensure that they stick to the filling.

Make another layer in the same way.

Serve cold or at room temperature.