In a large bowl, combine 2 1/2 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of kosher salt. Whisk these dry ingredients together until well blended.

Add the beaten egg, 1/2 cup of vegetable oil, 1 teaspoon of lemon juice, and 1/2 cup of water to the flour mixture, stirring until a smooth dough forms.

In a separate pot, boil 1 pound of russet potatoes until tender, about 15-20 minutes. Drain them well and mash until smooth, allowing to cool slightly.

Sprinkle in kosher salt and freshly ground black pepper to the mashed potatoes, adjusting to your taste.

In a skillet over medium heat, melt 2 tablespoons of butter and sauté 1/2 cup of finely sliced scallion whites and greens until softened, about 3-4 minutes.

Combine the sautéed scallions with the mashed potatoes in a bowl, ensuring everything is well mixed. Then, stir in the beaten egg.

On a floured surface, roll out the dough to about 1/4 inch thickness. Cut the dough into circles or squares.

Place a generous spoonful of the potato filling in the center of each piece of dough. Carefully fold over and seal the edges.

Arrange the filled knishes on a baking sheet lined with parchment paper and brush the tops with a little beaten egg.