Potato Knishes - Master recipes

Ingredients

• For the dough -
• 2 1/2 cups all-purpose flour -
• 1 teaspoon baking powder -
• 1/2 teaspoon salt -
• 1/2 cup water -
• 1/2 cup neutral oil (like vegetable or canola) -
• 1 large egg, whisked -
• 1 teaspoon white vinegar -
• For the filling -
• 3 tablespoons olive oil -
• 1 medium yellow onion, chopped -
• 1/4 teaspoon salt -
• 1/2 teaspoon black pepper -
• 1 pound russet potatoes, peeled and quartered -
• 2 tablespoons unsalted butter -
• For the egg wash and serving -
• 1 tablespoon water -
•  Mustard, for serving -