How to Make It
In a medium bowl, combine the mashed potatoes and turkey. Using a 2 ½-inch round cookie cutter, cut 24 circles from the lasagna sheets. Bring a large pot of water to a boil.
Working with one round of dough at a time, place a heaping tablespoon of filling in the lower half of the circle, leaving a ½-inch border. Wet the opposite edge of the round with a wet finger and fold it over filling. Pinch the edges together to form a tight seal. Repeat with the remaining dough rounds and filling.
Working in two batches, boil the pierogies until they rise to the top of the pot, about 4 minutes, then use a slotted spoon to transfer them to a paper towel lined baking sheet.
In a large skillet, melt half of the butter over medium heat. Once butter has melted, add the sage leaves and cook until they are fragrant and crisp, about 2 minutes. Use a slotted spoon to transfer the leaves to paper towel lined baking sheet. Immediately add half of the pierogies to the pan and cook, flipping once, until lightly browned on both sides, about 2 minutes. Transfer pierogies to a serving platter, wipe pan clean and repeat with remaining butter and pierogies. Top with crispy sage leaves and serve immediately.