Mix flour and salt, then rub in 100g chilled butter until the mixture resembles breadcrumbs.
Gradually add 60-90ml water to form a dough and roll into a rectangle on a floured surface.
Grate 200g frozen butter over the bottom two-thirds of the dough and fold into thirds like a letter.
Turn the dough 90 degrees and roll it out into a rectangle again.
Fold in thirds, wrap in film, and chill for 30 minutes; repeat this process two more times for four total turns.
Preheat the oven to 200°C.
Pulse mushrooms, salt, pepper, and thyme in a food processor to form a rough paste.
Cook the mushroom paste in a dry pan over medium-high heat until moisture evaporates, then cool.
Sauté onions in oil and butter with sugar for 20 minutes until soft, then stir in vinegar.
Roll pastry to a 60x45cm rectangle and cut into 12 squares.
Spread mushroom paste down the centre of each square, leaving a 2cm gap at the ends.
Mix sausage meat and black pudding, shape into 12 cylinders, and place over mushrooms topped with onions.
Cut corner triangles, fold ends over, and cut 1cm fringes along the sides to plait over the filling.
Place plaits on a tray, brush with egg, and sprinkle with sesame seeds.
Bake for 20 minutes until golden-brown and cool for five minutes before serving.