1 Preheat the oven to 400° F. In a medium bowl, combine the oil, water, and 1 teaspoon of the salt. Mix in 1 ½ cups of flour. Gradually mix in the remaining 1 cup of flour to make a soft dough that pulls away from the sides of the bowl. Shape into a ball, cover with plastic wrap and let rest at room temperature (do not refrigerate) for at least 30 minutes.
2 In the meantime, toss the sweet potatoes and garlic with 2 tablespoons of oil in a roasting or casserole pan, and roast in a preheated oven for 35–40 minutes, or until the garlic is golden brown and the sweet potatoes are fork tender.
3 Mash the sweet potatoes and garlic with a fork or potato masher.
4 Season the mashed sweet potatoes with the remaining ½ teaspoon salt, pepper, and garlic powder.
5 Lower the oven temperature to 375° F. On a floured surface, thinly roll out one piece of dough (about 1/8” thick). The dough will be slightly stretchy so use some elbow grease when rolling. Using the rim of a large drinking glass, or a round 3” cookie cutter, cut out circles from the dough. Collect the scraps, combine, and re-roll them to use up all the dough.
6 Spoon 1 heaping tablespoon of the sweet potato filling into the center of each dough circle. Gather the edges together around the filling and pinch closed to form a purse. Repeat with the remaining pieces of dough and filling.
7 Evenly space the knishes on a greased baking pan. Whisk together the egg yolk and water in a small bowl, and brush the knishes lightly with this egg wash. Bake for 35-40 minutes, or until golden brown. Serve warm with the mustard or horseradish sauce.