Stir flour and salt together in a large bowl. Whisk sour cream, eggs, egg yolk, butter, and oil in a separate bowl. Combine wet and dry ingredients to form dough. Cover with a towel and let rest for 15-20 minutes.

Place potatoes in a pot and cover with water. Bring to a boil and cook until tender, about 15 minutes. Drain and mash with shredded cheese and cheese sauce. Season with onion salt, salt, and pepper.

Divide dough into two portions. Roll one out to 1/8-1/4 inch thick. Cut into 3-4 inch circles using a cutter or glass.

Place filling in the center of each dough circle, brush edges with water, fold into half-moons, and press edges to seal tightly.

Freeze uncooked pierogies on a baking sheet; then store in bags or containers.