1. Preheat the oven to 400°F (200°C). Pulse the flour and walnuts together in a food processor until the nuts are very finely ground. Add the butter and pulse until it resembles coarse bread crumbs. Sprinkle in 4 tbsp of cold water and pulse until it clumps together, adding more water if needed.

2. Roll the pastry on a floured surface into a 1/8 in (3mm) thick round. Line the tart pan with the dough, trimming any excess. Prick the dough with a fork and refrigerate for 30 minutes.

3. Line the pan with parchment paper and baking beans. Bake for 10 minutes. Remove the paper and beans and bake for 10 minutes more until golden brown. Reduce the oven temperature to 350°F (180°C).

4. Whisk the cream, potato, eggs, and egg yolks together well. Add the cheese and season with the pepper and nutmeg. Pour into the pastry shell and sprinkle with the rosemary. Bake for 25 minutes, until set.

5. Let cool for 10 minutes, then carefully remove the sides of the pan. Slice the tart and serve warm.