In the bowl of a stand mixer fitted with a dough hook, blend together the flour and salt on low speed.

Whisk together the milk, egg and sour cream. Slowly add the milk mixture to the flour in a steady stream.

Continue to blend the flour mixture together until a shaggy dough forms and the dough starts to pull away from the sides of the bowl.

Cover the dough with plastic wrap and allow to rest for 1 hour.

Add the potatoes to a pot and fill with water to just cover the potatoes. Stir in the salt and bring the potatoes to a boil.

Turn the heat to low and continue cooking the potatoes until fork tender, approximately 20 minutes.

While the potatoes are cooking, melt the butter in a large skillet over medium heat. Add the onions to the pan. Cook until caramelized and golden brown, stirring occasionally, approximately 20 minutes. Set aside 1/4c of caramelized onions for later.

Drain the potatoes and transfer to a large bowl.

Add the rest of the onions, cottage cheese and cream cheese to the potatoes. Mash using a potato masher until well blended.

Season with salt and pepper, to taste.

On a lightly floured work surface, roll out the dough to approximately 1/4 inch thick. Cut out rounds with a 2 3/4-inch cutter. Place on a baking sheet, and cover with plastic wrap.

Using your hands, lightly flatten the dough disk to stretch it out a bit.

Place approximately 1 1/2 tablespoons potato filling in the center of the dough and fold the top half down to close, gently pressing to squeeze out any air and seal by pinched the seam together. Avoid overfilling.

Return to baking sheet and cover.

Repeat with remaining dough and filling.

Bring a large pot of salted water to a boil. Add 4 dumplings. Cook, stirring gently to avoid sticking to the bottom of the pot, until dumplings rise to the surface, about 4 minutes. Using a slotted spoon, transfer to a plate lined with paper towels, and pat dry. Transfer to a serving platter, and cover with foil to keep warm.

Pat pierogi dry with paper towels.

Heat a skillet over medium heat with 3 TBS olive oil and 1 TBS butter until butter is melted. Add 4 pierogi at a time, turning over until crisp on all sides. Remove pierogi from skillet onto a plate.

Turn off heat and add sage leaves, swirling around the skillet until crisp, about 1 minute. Pour on top of pierogi and serve.