Step #1 In a heatproof bowl, soak the morels in the boiling water until softened, about 20 mins.
Step #2 Rub them to loosen any grit, then lift them out & coarsely chop.
Step #3 In a large bowl, mix the flour with 1/2 tsp of salt.
Step #4 Add the eggs & stir until the dough just comes together.
Step #5 Transfer to a lightly floured work surface & knead briefly until soft & smooth.
Step #6 Wrap the dough in plastic & set aside at about room temp for 30 mins.
Step #7 Meanwhile, cook the potatoes In a large-ish saucepan of boiling salted water until tender, about 20 mins; drain.
Step #8 Return the potatoes to the pan & shake over high heat to dry them, about 1 min.
Step #9 Using a potato masher, mash the potatoes.
Step #10 Mash in the milk, sour cream, Parmesan & butter.
Step #11 Stir in the morels & season this with salt & pepper.
Step #12 Transfer to a bowl & let cool slightly.
Step #13 Cut the dough in half & pat each piece into a disk.
Step #14 On a well-floured work surface, roll out the disks to 9-inch rounds.
Step #15 Cover up with plastic wrap & let this stand for 15 mins.
Step #16 On a floured work surface, roll out 1 round at a time to a 1/16-inch thickness.
Step #17 Using a 4-inch biscuit cutter, stamp out 9 smaller rounds.
Step #18 Lightly moisten the edge of 1 round & dollop 1 rounded tbsp of the potato filling on half of it.
Step #19 Pat the filling slightly; fold the dough over the filling to form a half moon & press the edges to seal.
Step #20 Set aside on a floured platter.
Step #21 Repeat with the rest of the dough & filling.
Step #22 Cook half the pierogies at a time In a large-ish pot of boiling salted water until they rise to the surface, about 1 min.
Step #23 Using a slotted spoon, transfer them to paper towels & dry.
Step #24 In a large bowl, toss the pierogies with just enough vegetable oil to coat.
Step #25 Light a grill or heat a grill pan.
Step #26 Grill the pierogies over a hot fire, turning once, until browned, about 3 mins per side.
Step #27 Remove from the grill.
Step #28 Add the celery leaves & grill until lightly charred, about 1 min per side.
Step #29 Transfer the pierogies to a platter & garnish with the grilled celery leaves.
Step #30 Make Ahead: The pierogies can be prepared through Step 5 & frozen for up to 1 month.
Step #31 Boil the frozen pierogies for the same amount of time indicated for the fresh ones.
Step #32 Wine Recommendation: The rich, smoky filling in these pierogies makes them perfect foil wraps for a full-flavored Merlot with velvety tannins like the 1994 Columbia Crest Estate Series from Washington State or the 1995 Meerlust from South Africa.