Potato and Cheese Pierogi

Name: Potato and Cheese Pierogi
Ingredients: For the Filling:; 5 tablespoons (75g) unsalted butter, divided; 1 large onion (10 ounces; 285g), finely chopped; Kosher salt and freshly ground back pepper; 1 pound (455g) russet potatoes, peeled and cut into 1-inch pieces; 6 ounces (170g) farmer cheese; For the Dough:; 12.5 ounces (about 2 1/2 cups; 355g) high-protein all-purpose flour, such as King Arthur (see note); 1 teaspoon (4g) Diamond Crystal kosher salt; for table salt use half as much by volume or an equal amount by weight; 8 ounces (1 cup; 225g) sour cream; 1 large egg; To Assemble and Finish:; 2 tablespoons (30ml) vegetable oil, divided; 2 tablespoons (30g) unsalted butter, divided; Sour cream, for serving; Sliced chives, for serving (optional)