In a large bowl, beat together the ingredients for the dough. Lightly flour a flat surface and knead the dough for a few minutes until it forms a smooth, elastic dough. Cover the dough in plastic wrap and let it rest for at least 30 minutes.
While the dough rests, bring a large pot of water to a boil. Add the peeled sweet potatoes to the boiling water. Boil for about 15-20 minutes, until the sweet potatoes are fork-tender.
Once the sweet potatoes are cooked, mash them using a ricer or fork. Mix in the Greek yogurt and butter. Stir in the rosemary, paprika, onion powder, garlic powder, and brown sugar. Allow the mixture to cool until it is no longer steaming.
As the filling cools, roll out the dough on a surface dusted with flour to about 1/8 inch thickness. Use a Mason jar ring or a wide mouth glass to cut the dough into individual circles.
Spoon about a tablespoon of the filling onto the center of each wrapper. Fold the wrappers in half and use a touch of water to help seal the edges. Place on a paper towel as you prepare the rest of the pierogies.
Bring a large pot of water to a boil again. Add the pierogies in batches and boil for 1-2 minutes, until they begin to float. Remove from the water and place back onto a plate lined with a paper towel.
In a large skillet, melt two tablespoons of butter. Add the boiled pierogies to the hot butter in batches. Sauté until golden brown, or to the desired level of crispiness. Set aside the pierogies to make the brown butter sauce.
In the same skillet, melt the remaining butter. Add the sliced red onions and sage. Sauté for about 10 minutes, until the onions have softened but have not lost their color. Pour the sauce over the cooked pierogies. Serve hot and enjoy!