Combine flour and butter in a bowl and, using your fingertips, rub in butter until mixture resembles fine crumbs. Add 1 egg yolk (reserving eggwhite) and 1-2 tbsp of cold water and mix to just combine, then turn onto a lightly floured work surface and knead lightly until smooth. Wrap in plastic wrap and refrigerate for 1 hour to rest.
Preheat oven to 180C. Roll out pastry on a lightly floured surface to 3mm-4mm thick and line a 24cm-diameter loose-bottomed fluted tart pan, trimming edges and reserving scraps. Refrigerate scraps and tart for 30 minutes.
Combine remaining eggs and sugar in a bowl and stir to combine. Stir in hazelnuts, then spoon into tart case (there may be some filling leftover). Roll out pastry scraps on a lightly floured surface to 3mm-4mm thick, cut into 2cm-wide strips and arrange in a lattice pattern over filling. Brush with lightly beaten reserved eggwhite and bake until golden (about 30 minutes). Serve immediately or at room temperature with sheep’s milk yoghurt.