Preheat oven to 170°C (fan forced).
Lightly flour your work surface and lay out the sheet of pastry. Only take the pastry out of the fridge when you’re ready to start so it doesn’t dry out.
Cut pastry into 4 large triangles.
Layer 2 slices of mushroom and 1 small wedge of Manchego at the base of the triangle. You can also experiment with your favourite type of ham and cheese, or other ingredients in this step.
Starting where you have placed the filling, roll the pastry towards the opposite point to form a cigar. Repeat for each triangle.
Place each croissant, seam side down, on a lined baking sheet and lightly coat with egg wash.
Bake in the oven at 170°C for 15 minutes until golden.
If you’re entertaining, you can prep your croissants beforehand and keep them in the fridge until your guests arrive. When you’re ready, add the egg wash and pop them in the oven.