Make dough: Stir together dry ingredients in a bowl. In a small bowl, whisk together egg, oil, vinegar, and water. Pour over dry ingredients and stir to combine. Knead until smooth, cover, and set aside for an hour.
Prepare filling: Boil potatoes until soft, about 20 minutes. Drain and cool. In a skillet, heat butter and oil, add onions, and cook until caramelized, about 45 minutes. Mash potatoes with onions, salt, and pepper.
Assemble knish: Roll out dough, create a log with filling, and wrap dough around it. Seal ends and indent the dough.
Bake knish: Preheat oven to 375°F. Brush with egg yolk mixture and bake for about 45 minutes until golden brown.
