Peel and cube 4 medium potatoes to ensure even cooking.

Boil the potatoes in salted water until tender, about 15 minutes; drain well.

Heat 2 tablespoons vegetable oil in a pan over medium heat and sauté 1 large chopped onion until golden brown, about 5–7 minutes.

Mash the drained potatoes in a bowl until smooth and stir in the sautéed onions; season with salt and pepper to taste.

In a separate bowl, combine 3 cups flour and 1 teaspoon salt; cut in 1 cup softened butter until mixture resembles coarse crumbs.

Add 6 tablespoons cold water gradually, mixing until a dough forms; knead gently on a floured surface until smooth.

Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

Roll out the dough and cut into squares; place a few tablespoons of potato filling in the center of each square, fold and seal the edges.

Brush each knish with beaten egg and bake for 30 minutes or until golden brown.