Red Potato Knish with Kale, Leeks and Cream Cheese

From SmittenKitchen.com

Dough
2 1/2 cups all-purpose flour1 teaspoon baking powder1/2 teaspoon table salt1 large egg1/2 cup vegetable oil1 teaspoon white vinegar1/2 cup water 
1 big leek (about 1/2 pound), white and light-green parts only, halved lengthwise, and thinly sliced (you’ll clean the grit out in a moment)
1 tablespoon vegetable or olive oil
1 tablespoon unsalted butter
1 generous cup lacinato kale ribbons (about 3 ounces or 1/4 to 1/3 bundle), tough stems and ribs removed and leaves cut into strips (you’ll wash it in a moment)
1/2 teaspoon table salt
Freshly ground black pepper
3 ounces cream cheese, at room temperature
1 large egg yolk
1 teaspoon water

Filling

1 1/2 pounds medium red potato (about 3 to 4), peeled and quartered

To finish

Make dough: Stir together your dry ingredients in the bottom of a medium/large bowl. In a small bowl, whisk together the egg, oil, vinegar and water. Pour it over the dry ingredients and stir them to combine. Once the mixture is a craggy, uneven mass, knead it until smooth, about a minute. Place the dough back in the bowl and cover it with plastic wrap. Set it aside for an hour (or in the fridge, up to 3 days) until needed.

Cook potatoes: Put potatoes into a large pot, cover with cold water, and bring to a boil over high heat. Reduce heat to medium and cook until soft, about 20 to 25 minutes. Drain, then transfer to a large bowl to cool.

Meanwhile, prepare leeks and kale: Fill a medium bowl with very cold water and drop in leek rings. Swish them around with your fingers, letting any sandy dirt fall to the bottom. Scoop out the leeks and drain them briefly on a towel, but no need to get them fully dry. Do the same with the kale, but you can leave the leaves to nearly fully dry, patting them if necessary, on the towels while you cook the leeks.

Heat a large, heavy skillet over medium heat. Once hot, add butter and oil and once they’re fully melted and a bit sizzly, add the leek slices. Reduce heat to low, cover with a lid and cook leek for 20 minutes, stirring occasionally. Raise heat back to medium, add the kale ribbons and cook until they wilt, about 5 minutes.

Transfer mixture to bowl with potatoes, add the cream cheese and mash together until combined. Stir in salt and many grinds of black pepper and set filling aside.