In a large mixing bowl, combine the gluten-free flour blend and salt. Gradually add water and eggs, mixing until a cohesive dough forms. Knead lightly for about 5 minutes and refrigerate for at least 30 minutes.

Bring a pot of salted water to a boil, add chopped russet potatoes, and cook for 15-20 minutes until fork-tender. Drain and mash until smooth.

In a skillet over low heat, melt butter and cook diced onions for 20-25 minutes until golden brown.

Combine mashed potatoes, sautéed onions, and cheddar cheese in a large bowl. Season with salt and pepper to taste.

On a floured surface, roll out dough to about 1/8 inch thick. Cut circles, fill with a teaspoon of filling, and fold to create a half-moon shape.

Boil assembled pierogi in salted water until they float (5-6 minutes), then transfer to a plate.

Heat butter in a skillet and fry boiled pierogi for 3-4 minutes on each side until golden brown.

Plate pierogi hot, garnish with sour cream or applesauce, and optional toppings like chives or bacon.