Preheat your oven to 190C / 170C Fan / Gas 5.

Cut two pieces of baking paper to fit the bases of two baking trays each approximately 31 x 21 cm.

Stir together the flour and salt in a bowl.

Grate the butter into the flour (dip the butter in the flour now and then to stop it sticking to the grater) then rub in with your fingers until you have a mixture resembling coarse breadcrumbs.

Add just enough iced water to create a soft, pliable dough: start by mixing in the water with a knife, then switch to your hands.

Cover and leave to rest for 5 minutes.

Meanwhile, finely grate both cheeses and mix together.

On a floured work surface, roll out the dough into a rectangle approximately three times as long as it is wide and 3-5 mm thick.

With one of the short sides facing you, sprinkle one third of the cheese onto the lower two-thirds of the dough.

Fold down the top third of the dough, then fold down again so the cheese is completely covered.

Give the dough a quarter turn so that the folded sides are now to the left and right.

Roll into a rectangle as before, then repeat the process of sprinkling the lower two thirds with cheese and folding over again.

Again, turn so that the folded sides are to left and right, then roll out and repeat with the final third of cheese.

With your rolling pin, lightly press down the open sides of the dough to stop any cheese falling out.

Cut the dough into two pieces and cover one of them.

Lightly flour one of the pieces of baking paper.

Place one piece of dough on it, sprinkle with flour, and roll out 2-3 mm thick or until it covers the paper.

Neaten the edges of the dough with a knife or pastry wheel.

Cut the dough into squares or rectangles of your preferred size: try not to cut through the paper.

Tip: using a ruler width as a template, you should get 5 rows of 8 i.e. 40 small square crackers per tray.

Slide the paper and the uncooked crackers onto one of the baking trays.

Brush the dough with beaten egg and sprinkle over your chosen toppings.

Prick the crackers with a fork.

Can be chilled or frozen at this point: see below.

Put the tray in the oven and bake until the crackers are puffed up and golden brown (12-15 min).

While the first tray is baking, make another batch with the remaining dough then cook as above.

When done, transfer the crackers to a wire rack to cool a little.

Eat warm or cold.

Store in an airtight container when completely cold.

Best eaten on the day of making while still crispy but can be re-crisped in the oven: 2-3 minutes, temperature as above.

You can also prepare in advance and chill or freeze after Step 7. No need to defrost, just bake from frozen (or chilled) as per original baking instructions.