In a large mixing bowl, combine 3 cups of all-purpose flour, 1 teaspoon of baking powder, and 1 teaspoon of kosher salt. Add ¼ cup of vegetable oil, 1 cup of warm water, 1 lightly beaten egg, and optionally, 1 teaspoon of white vinegar. Mix until combined and form a dough. Knead gently until smooth for about 5 minutes, then let it rest by chilling in the refrigerator for 30 minutes.

While the dough chills, prepare the filling by peeling and cutting 2 large russet potatoes into chunks. In a large pot, bring salted water to a boil, then add the potatoes and cook until tender, approximately 15-20 minutes. Drain well and return the potatoes to the pot, then mash them until smooth and fluffy.

In a skillet, heat 2 tablespoons of extra-virgin olive oil over medium heat. Add 1 chopped yellow onion and sauté for about 8-10 minutes until soft and caramelized. Combine with the mashed potatoes, stirring in ½ cup of full-fat sour cream and freshly ground black pepper to taste.

Once the dough has chilled, divide it into 4 equal portions on a lightly floured surface. Roll out each portion into a thin sheet, about 1/8 inch thick. Cut the dough into 4x4-inch squares.

Place a generous spoonful of the potato filling in the center of each dough square. Fold the square diagonally to form a triangle and pinch the edges firmly to seal.

Preheat your oven to 375°F (190°C). Lightly grease a baking sheet with cooking spray or parchment paper. Arrange the filled knishes on the baking sheet and brush the tops with a little beaten egg.

Place the baking sheet in the preheated oven and bake the knishes for 30-40 minutes until golden brown and crispy.

Once baked to perfection, remove the knishes from the oven and allow them to cool on a wire rack for about 5 minutes. Serve warm with a side of creamy dill yogurt sauce or your favorite condiment.