Caramelized Onion and Potato Knishes

Name: Caramelized Onion and Potato Knishes
Ingredients: 2 1/2 cups all-purpose flour, plus more for dusting (see Cook's Note); 1 teaspoon baking powder; 1 teaspoon kosher salt; 6 tablespoons schmaltz, melted and cooled, or neutral oil, such as vegetable oil (see Cook's Note); 1 teaspoon distilled white vinegar; 2 tablespoons schmaltz or neutral oil, such as vegetable oil (see Cook's Note); 2 large yellow onions, cut into a medium dice; Kosher salt and freshly ground black pepper; 1 tablespoon chopped thyme leaves (from about 12 sprigs); 2 garlic cloves, minced; 1 1/2 pounds russet potatoes, cut into medium chunks; 5 ounces cream cheese, at room temperature; 1/3 cup sour cream, plus more for serving; 1/4 cup dill fronds, chopped; 2 tablespoons minced chives (from 4 to 6 chives); Nonstick cooking spray; 1 large egg