French Onion Soup Pierogi

**Prepare the Potatoes:** Boil the potatoes in a large pot of salted water until tender, about 15-20 minutes. Drain well and return to the pot.

**Make the Potato Filling:** Mash the potatoes with a potato masher or electric mixer until smooth. Stir in the shredded cheddar cheese, softened cream cheese, salt, and pepper. Set aside to cool slightly.

**Caramelize the Onions:** Heat the butter and olive oil in a large skillet over medium-low heat. Add the sliced onions and cook slowly, stirring occasionally, until they are deeply caramelized and golden brown, about 30-40 minutes. Stir in the chopped thyme during the last 5 minutes of cooking. Season with salt and pepper to taste. Reserve about 1/2 cup of the caramelized onions for serving.

**Assemble the Pierogi:** Roll out the pierogi dough on a lightly floured surface. Cut out circles using a cookie cutter or glass. Place a spoonful of the potato filling in the center of each circle. Fold the dough over to form a half-moon shape and crimp the edges to seal tightly. Repeat until all the filling is used.

**Cook the Pierogi:** Bring a large pot of salted water to a boil. Add the vegetable oil. Gently drop the pierogi into the boiling water, being careful not to overcrowd the pot. Cook for 2-3 minutes, or until they float to the surface. Remove with a slotted spoon and drain well.

**Serve:** Serve the pierogi hot, topped with the remaining caramelized onions and a dollop of sour cream.