1: To make the potato filling, bring a medium saucepan of salted water to the boil. Add the diced potatoes and cook for around 10-1 minutes until tender. Drain in a colander and leave for a few minutes to allow the potatoes to steam and dry out.
2: Meanwhile, heat a frying pan over a medium heat and add the oil. When it is hot, add the onion and garlic and sweat for around 10-12 minutes until soft and deep golden – you want the onion to be quite caramelised.
3: Transfer the onion and garlic to a large bowl, add the potatoes and mash together well until smooth. Stir in the matzo meal, sugar and beaten egg. Season generously with salt and pepper and mix well to combine. Cover with cling film and place in the fridge to chill.
4: To make the dough, put the flour, baking powder, salt, oil, egg and water in a freestanding electric mixer fitted with the dough hook (or into a food processor), and mix to a soft, pliable dough. Wrap the dough in cling film and chill in the fridge for an hour.
5: Preheat the oven to 180C/Fan 160C/Gas 4. Lightly grease a large baking tray.
6: Using a lightly floured rolling pin, roll the dough out as thinly as possible on a floured surface to a 30cm square.
7: Spoon the potato filling into the centre of the dough and spread it out evenly, leaving a 2-3cm clear margin. Brush the edges of the dough with egg wash, then roll up tightly like a swiss roll. Press the ends together with your fingers to seal.
8: Gently press and pinch the open edges to seal the buns, shaping them gently with your hands. Place the buns flat side down on the prepared baking tray and brush each one generously with egg wash.
9: Bake for 40-50 minutes, until golden brown and crisp.
10: Serve the knish warm, or on their own as a snack, or floating in bowls of soup or stew.