Philly Cheesesteak Pierogi Recipe

Ingredients (12)

• 2 1/4 cups all-purpose flour, plus extra for dusting
• 1 teaspoon kosher salt, plus more as needed
• 1 large egg, beaten
• 1 cup sour cream, plus more for serving
• 6 tablespoons unsalted butter, softened, divided
• 1 large yellow onion, thinly sliced
• 3 tablespoons canola or vegetable oil, divided, plus more as needed
• 10 ounces thinly sliced steak, such as ribeye or top round (see note)
• Freshly ground black pepper
• 1 1/2 cups shredded provolone cheese
• 1 cup shredded low-moisture mozzarella cheese
• Chopped flat-leaf parsley, to garnish