Method

• 1.
  Place the potato in a saucepan of cold, salted water and bring to a simmer over medium heat. Simmer for 10-15 minutes or until tender. Drain, then leave in the colander to steam-dry for a few minutes. Return to the saucepan and mash with 80g butter. Season, and set aside to cool.
• 2.
  Preheat the oven to 180°C. Grease 2 baking trays and line with baking paper.
• 3.
  For the dough, pour 150ml water into a large mixing bowl. Using a fork, stir in the oil, vinegar, egg and 1 tsp salt flakes. Gradually add the flour, bringing it together with your hands until a sticky dough forms. Tip out onto a clean, lightly floured work surface and knead for 5 minutes or until dough is smooth. Place in a lightly floured bowl, cover with a damp tea towel and set aside.
• 4.
  Heat the oil and remaining 20g butter in a large frypan over medium heat. Add the spring onion, garlic and three-quarters of the thyme and cook, stirring, for 5 minutes or until soft. Add mushroom and cook for 15-20 minutes, stirring often, or until softened.

• 5.
  Combine the mushroom mixture with the potato, then add the parsley and cheddar, and stir until combined. Taste, adjusting seasoning if necessary.
• 6.
  Divide into 12 and roll into balls using your hands. Place on a clean work surface and flatten into patties.
• 7.
  Divide the dough into 4 even pieces. Place 1 piece onto a lightly floured surface and roll out into a 30cm x 30cm square. Cut into three 10cm-wide strips. Place a patty 5cm from the end of the strip, then stretch the end of the dough over the top to cover, tucking it in to seal. Roll up the patty in the strip, twisting it slightly with each roll so none of the filling is exposed. Continue to tuck and roll the dough until you reach the end and the patty is evenly sealed. Repeat with remaining dough and patties until you have 12 knishes.
• 8.
  Place on the prepared trays, brush with beaten egg and sprinkle with the parmesan and remaining thyme leaves. Bake for 25-30 minutes or until golden and cooked through.
• 9.
  Leave to cool for a few minutes before serving.