To make the filling, combine the mashed sweet potatoes, ricotta cheese, parmesan cheese, nutmeg, 0.5 tsp salt, and pepper in a bowl. Mix until smooth and set aside.

To make the dough, pile the flour on a clean surface and make a well in the center. Crack the egg into the well, add the oil, warm water, and remaining 0.5 tsp salt.

Gently mix the wet ingredients into the flour until a shaggy dough forms. Knead the dough for about 5-7 minutes until soft and elastic. Cover with a towel and let rest for 20 minutes.

Roll the dough out on a floured surface until it is about 1/8 inch thick. Use a round cutter or glass to cut out circles approx 3 inches in diameter.

Place a spoonful of the sweet potato filling in the center of each circle. Fold the dough over the filling to create a half-moon shape and pinch the edges tightly to seal.

Bring a large pot of salted water to a boil. Drop the pierogi in batches and cook for 3-4 minutes, or until they float to the top. Remove with a slotted spoon.

In a large skillet, melt the butter over medium heat until it starts to foam and turn golden brown (about 3 minutes). Add the sage leaves and fry for 30 seconds.

Add the boiled pierogi to the skillet and toss gently to coat in the brown butter. Pan-fry for 2-3 minutes per side until slightly crispy. Serve hot.