Potato Knish

Name: Potato Knish
Prep time: 4 h 30 min
Cook time: 1 h
Total time: 5 h 30 min
Cuisine: European
Ingredients: 2 and ½ cups (350 g) all-purpose flour; 1 tsp. baking powder; ½ tsp. fine sea salt; 1 egg; 1 tsp. white vinegar; ½ cup neutral oil such as vegetable or canola; ½ cup warm water; 2 lbs. Russet or baking potatoes, peeled and cut into 2-inch chunks; 3 tsp. kosher salt, divided; 3 Tbsp. butter, oil, or schmaltz*; 2 yellow onions, thinly sliced; ½ cup scallions, sliced; ½ tsp. white pepper; 1 egg beaten with 1 Tbsp. of water