1. Put the flour, salt, mustard powder, paprika and butter into a food processor and whizz to combine together. Add the grated cheeses and sprinkle with one tablespoon of iced water, then pulse until the pastry comes together.
2. Divide the pastry into two pieces and shape into logs. Roll the logs up in parchment paper and chill in the fridge for at least 30 minutes.
3. Preheat the oven to 180°C/160°C fan/ gas mark 4.
4. Unwrap the logs and brush with egg. Place the chopped walnuts on a plate or board and roll the logs in the walnuts, pressing them on gently so they stick.
5. Cut the logs into 5mm discs and place on a lined baking tray. Brush the tops with egg wash and sprinkle with the rosemary and salt flakes.
6. Bake for 10-15 minutes until golden. Allow to cool completely, then store in an airtight container.

Per serving: (3 biscuits) 118kcals, 8.6g fat (5.3g saturated), 7g carbs (0.1g sugars), 3.4g protein, 0.3g fibre, 0.178g sodium