Make the Dough
In a large bowl, combine the flour, baking powder and salt.
In a small bowl, whisk together the water, oil, egg and vinegar, then stir into the flour mixture until you have a rough dough.
Knead in the softened butter for about 5 minutes or until the dough is smooth. Wrap in cling wrap and refrigerate while you prepare the filling.
Make the Filling
Bring 2 inches (5 cm) of water to a simmer in a large pot. Insert a steamer basket with the potatoes. Place the lid on the pot and steam the potatoes for about 30 minutes, or until tender. Keep in the basket to stay warm until ready to use.
Meanwhile, combine the onions, oil, butter and salt in a frying pan and cook over low heat, stirring occasionally, until the onions caramelize and turn golden brown, about 30-40 minutes. Turn off the heat and stir in the thyme and pepper.
Add the potatoes to the onions in the frying pan and mash them until lump free. Taste and adjust the seasoning if needed.
Assemble and Bake the Knishes
Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper. Set aside.
Working with ½ of the dough at a time, roll out on a floured surface into a 6x12 inch (15x30 cm) rectangle.
Arrange the rectangle so that a 12-inch (30 cm) edge is closest to you. Brush the long edges with the egg wash.
Place ½ of the filling about 1-inch away from the long edge closest to you and spread it out so it covers the whole length of the dough and is about 2-inches (5 cm) wide.
Starting at the edge closest to you, begin to roll the dough around the filling until you have a 12-inch (30 cm) log. The filling should come all the way to either edge of the log.
Using a ruler and your finger, dent the log into 6 equal pieces that are 2-inches (5 cm) in length. Then use a knife to cut the log where the dents were made.
For each knish, place one cut side down on the parchment. Pinch the top dough together, pressing down on the filling a bit so it bulges out a little, then twist the gathered dough to seal the top. Repeat with the remaining knishes and then the remaining dough and filling.
Brush all over with the egg wash and bake for 40-45 minutes, until deep golden brown. Serve warm or room temperature with mustard or sour cream.
Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat before serving in a 300°F (150°C) oven for about 15 minutes.