1 cup (3.75 oz) corn flour or sorghum
3/4 cup (3.75 oz) quinoa flour
3/4 cup (3.25 oz) potato starch
1 Tbsp xanthan gum
1 1/2 tsp guar gum
1 1/2 tsp salt
6 egg yolks
2 Tbsp neutral oil
2 onions, chopped (1 for filling, 1 for topping)
1 1/2 lb russet potatoes
1/2 cup grated cheddar
1/2 tsp salt
1/2 tsp pepper
To make the filling: Heat the oil in a large skillet over a low flame. Sauté the onions in the oil until golden brown, about 15-20 minutes. While the onions are cooking, peel the potatoes and cut into 1-inch cubes. Cook potatoes in a large saucepan of boiling salted water until quite tender, about 10 minutes. Drain the potatoes, and mash until smooth. Mix in half the caramelized onions (set aside the other half) and the cheddar cheese, and season with salt and pepper. Cover and keep the filling refrigerated until you are ready to fill your pierogie (can be prepared the night before).
To make the dough: Mix together all of the dry ingredients in the bowl of a stand mixer fitted with a paddle attachment. Add the eggs and yolks, and mix until the it forms a cohesive, firm dough. This should take a few minutes. If it doesn't come together, add an additional egg yolk. The resulting dough will be firm but easily moldable.
To finish the pierogies: Set a pot of salted water to boil over the stove. Pinch off a small amount of dough, and cover the rest so that it doesn't dry out. On a gf flour-dusted counter, roll the dough as thin as you can, about 1/8" or thinner (alternately, you can feed this through a pasta machine). If you favor traditional small pierogies, cut out circles with a 2" cutter. If you are lazier (like me), a slightly larger cutter works fine as well. Pull up the dough scraps, mold them together, and set aside with the remaining dough. Place a small amount of filling in the center of the circle, ~1 tsp for a 2" circle. Moisten the edges of the circle with a bit of water, fold in half, and crimp closed with a fork. Repeat with remaining dough and filling.
As the batches of pierogies are finished (or at the end, if you're working solo), drop them gently into the boiling water, and simmer until done, ~8 minutes. Remove with a slotted spoon or skimmer, and toss with a bit of butter to keep them from drying out.
To serve, take your reserved caramelized onions, and add a few tablespoons of water. Bring up to a boil for a minute or so, to soften the onions and make them saucy. Serve the pierogies topped with the caramelized onions and a good blob of sour cream.