Ham Cheese and Onion Empanadas

Name: Ham, Cheese, and Onion Empanadas
Ingredients: ½ cup (1 stick) unsalted butter, melted, slightly cooled; 1 Tbsp. apple cider vinegar; 1 Tbsp. kosher salt; 6 cups all-purpose flour, plus more for surface; 1 Tbsp. extra-virgin olive oil or unsalted butter; 2 medium onions, chopped; Kosher salt, freshly ground pepper; 1 Tbsp. dried oregano; 14 oz. uncured slow-cooked ham, coarsely chopped; 1 lb. whole-milk, low-moisture mozzarella, cut into ½ pieces; 1 Tbsp. cornstarch; Vegetable oil (for frying; about 6 cups); A 4½-diameter cookie cutter; a deep-fry thermometer