Rub the 50g of butter cubes into the flour and salt until the mixture resembles breadcrumbs.
Stir in 150ml of cold water gradually until a soft dough forms.
Pat the dough into a 2cm thick rectangle and roll out to 30x20cm.
Roll the remaining butter block between greaseproof paper to a 15x20cm rectangle.
Fold the pastry edges over the butter block and seal the edges by pinching.
Fold the pastry in half lengthways and rotate 90 degrees.
Roll the pastry out again to a 30x20cm rectangle.
Fold the edges to the centre, fold in half lengthways, and rotate 90 degrees.
Repeat the rolling process once more, wrap in film, and chill for one hour.
Sauté chopped onions and garlic in 25g of butter for 4-5 minutes.
Add brandy, ignite the alcohol, and allow flames to die down before cooling the mixture.
Mix the cooled onion mixture with wild boar, tarragon, salt, and pepper.
Preheat the oven to 230°C and line a baking tray with parchment.
Roll chilled pastry to a 40x30cm rectangle approximately 5mm thick.
Cut the pastry lengthways into two 40x15cm strips.
Place a line of filling down each strip, slightly off-centre.
Brush edges with egg yolk, fold over, and crimp to seal.
Place seam-side down on the tray, glaze with egg, and bake for 20-30 minutes at 230°C.
Slice and serve the sausage rolls hot or cold.