Divide and cut with a knife each sheet of puff pastry into 8 equal squares. You should have 16 squares.
Smear the center of only 8 squares of dough with 1 tablespoon of Dijon mustard.
Place one or two slices of mozzarella on mustard, sprinkle with 1 tablespoon of Swiss cheese.
Cover each pastry squares with the remaining 8 squares from the other pastry sheet.
With a fork press the square edges to paste them together and close the square.
Place the filled squares on the baking sheet lined with parchment paper or a silicon surface and bake in the oven at 200°C for 15 - 20 minutes or until golden in colour.
Serve hot or warm with fresh vegetables (tomatoes, spring onions, lettuce).