Pre-heat the oven to 200'C.
Combine the grated gruyere with the paprika and cayenne, if using.
Beat the egg with the smoked Maldon salt. If you prefer to avoid using eggs, you can use tap water or 2 tablespoons of melted butter instead.
Roll out the puff pastry to 5mm thickness, if necessary.
Brush both sheets of pastry generously with the beaten egg.
Sprinkle one sheet with grated cheese, reserving about a third for garnish. Place the second sheet of pastry on top of the other, egg-washed side down.
Use a rolling pin to stick the two layers together: the final thickness of the pastry should not exceed 7mm.
If desired, brush the top layer of pastry with more egg, and garnish with the remaining gruyere.
Leave the pastry to rest in the refrigerator for at least 30 minutes.
Cut the pastry into strips and bake until golden brown, about 10 minutes.
Serve immediately, or keep in an airtight container for about 5 days.
The unbaked strips of pastry can be frozen for about 3 months.
«Try using different cheeses and spices to vary the flavours: Comte with curry spices; Cheddar with cumin and coriander; Gouda with caraway seeds.»