Combine flour and salt. Beat egg yolks until smooth. Blend yolks and sour cream with dry mixture. Refrigerate up to 1 hour.

Place dough on lightly floured surface and roll until less than 1/8” thick. Cut into 2” circles (use a drinking glass or coffee cup).

Combine filling ingredients and place 1 tsp. of filling off-center on each circle. Fold dough over to form half circles; seal with tines of fork. Set aside.

In a large pot, bring 12 cups salted water to boiling. Add 10 to 12 pierogis; cook 5-7 minutes. Remove cooked pierogi and drain on paper toweling. Transfer to buttered serving dish. Brush pierogi with melted butter between layers to keep from sticking. Repeat until all are cooked. Top pierogis with sautéed onions.

Bake covered until heated through, about 15 minutes at 350(F) degrees. Be careful not to overcook. Pierogis can also be gently fired in butter in a large skillet and tossed with sautéed onion.