1. In a food processor or mixer, put the flour, salt, mustard powder (if using) and chilli flakes and press the pulse button.
2. Add the chilled cubed butter and pulse until it forms breadcrumbs.
3. Add the cheese and a tiny bit of cold water (about 1/2 teaspoon) and pulse the mixture until it comes together.
4. Tip the ball out onto a lightly floured surface and quickly, but gently, bring the mixture together. Roll into a log around 5 cm wide before wrapping up in parchment paper and then cling film.
5. Pop in the fridge to chill down for a few hours.
6. Preheat the oven to 180°C/350°F/gas 4 and line 2 baking sheets with parchment paper.
7. Remove the dough from the fridge and slice into rounds about 1/2 cm thick.
8. Place the biscuits on the baking paper leaving a small gap in between them as they will spread a little.
9. Brush with beaten egg and sprinkle over a little sea salt and a few more chilli flakes (if you wish).
10. Bake for 12 - 15 minutes, or until light golden brown before removing from the oven and leaving them to cool on the trays. Store in an airtight container for a few days.