Potato Pierogis

Ingredients

• FOR THE DOUGH:
• 2-1/2 cups All-purpose Flour
• 1/4 teaspoons Kosher Or Sea Salt
• 2 Tablespoons Sour Cream, Room Temperature
• 1 whole Egg (lightly Whisked)
• 1/2 Tablespoons Softened Butter
• 1/2 cups Warm Water
• 1/2 cups Whole Milk, Room Temperature
• _____
• FOR THE FILLING:
• 2-1/2 pounds Medium Yokon Gold Potatoes, Peeled, Quartered And Boiled In Heavily Salted Water Until Fork Tender
• 1 whole Yellow Onion, Peeled And Grated
• 2 Tablespoons Butter
• 1 cup Shredded Sharp Cheddar Cheese
• 1/2 teaspoons Kosher Or Sea Salt
• 1/2 teaspoons Freshly Ground Black Pepper
• _____
• FOR SERVING THE PIEROGIS:
• 1 stick Butter
• 1 whole Large Onion, Finely Chopped