For the pastry, sift the flour and a pinch of salt into a large bowl. Grate in the frozen butter. Use a kitchen knife to mix until all the pieces of butter are coated with flour. Stir in 150ml cold water to form a soft dough, then bring together with your hands. Wrap in cling film and chill in the fridge for 30 mins.
Heat oven to 200C/180C fan/gas 6. On a lightly floured surface, roll out half the pastry to a long rectangle, about 9 x 45cm. Scatter half the filling ingredients along the pastry, towards one edge. Brush the other edge with egg and fold it over the filling, pressing the pastry edges to seal into a long sausage roll. Brush the top with more egg, then cut into 3cm-long rolls. Use scissors to snip little ‘V’ shapes in the top of each roll.
Repeat with the remaining pastry and filling, then arrange the rolls on baking parchment-lined baking trays. Bake for 15-20 mins until golden brown.