Make the filling: Place potatoes and 1 tablespoon salt in a large pot. Add cold water to cover by 1 inch. Bring to boil, then reduce heat and simmer 15 minutes until tender.

Mash potatoes: Drain potatoes and pass through potato ricer or mash until smooth. Immediately add 2 tablespoons butter, cheese, 1/4 teaspoon salt, and pepper. Mix well.

Cool filling: Spread mixture on a tray, cover with plastic wrap pressed directly onto surface. Cool 30 minutes at room temperature, then refrigerate 1 hour until cold.

Make dough: Heat water and 3 tablespoons butter until butter melts. Whisk flour and 1 teaspoon salt in large bowl. Make well in center, add butter mixture and beaten egg. Mix into shaggy dough.

Knead dough: Turn onto lightly floured surface and knead 5 minutes until smooth. Wrap in plastic and rest 30 minutes.

Roll and cut: Roll dough to 1/8-inch thickness. Cut into 3-inch circles. Gather scraps, re-roll and cut more circles.

Fill perogies: Place 1 tablespoon filling in center of each circle. Wet edges with water, fold over and pinch to seal firmly.

Make onion butter: Melt 2 tablespoons butter in skillet over medium heat. Add diced onion and 1/2 teaspoon salt. Cook 10-15 minutes until golden. Set aside.

Boil perogies: Bring 3 quarts salted water to boil. Add 8-10 perogies and cook 5 minutes until floating. Reserve 1 cup cooking water, then remove perogies with slotted spoon.

Finish with butter sauce: Melt 1 1/2 tablespoons butter in large skillet. Add cooked perogies, 1/3 of onion mixture, and 2 tablespoons reserved cooking water. Toss 1 minute until coated.

Serve: Transfer to serving plate with all butter sauce. Garnish with parsley and serve with sour cream.