Pour boiling water over porcini in a small bowl and soak until softened,10 to 20 minutes. Lift porcini out of water, squeezing excess liquid back into bowl, and rinse well to remove any grit.
Pour soaking liquid through a paper-towel-lined sieve into a bowl and reserve.
Finely chop onion and garlic in a food processor, then add cremini and porcini and pulse until very finely chopped.
Heat butter in a skillet over moderate heat until foam subsides, then cook mushroom mixture, stirring frequently, until mushrooms are dry and 1 shade darker, about 8 minutes.
Add reserved soaking liquid and simmer, stirring frequently, until mixture is thick, dry, and beginning to brown, about 15 minutes (there will be about 1 cup filling). Stir in parsley and salt and pepper to taste. Cool completely.
Halve dough and roll out 1 piece on a lightly floured surface into a 15-inch round, keeping remaining dough wrapped.
Cut out rounds (about 2
with floured cutter. Put 1 teaspoon filling in center of each round. Working with 1 round at a time, moisten edges with water and fold in half to form a half-moon, pinching edges together to seal.
Transfer pierogies as assembled to a flour-dusted kitchen towel. Repeat with remaining rounds, then make more pierogies with remaining dough and filling.
Cook onions in butter in a large heavy skillet over moderately low heat, stirring frequently, until golden brown, about 30 minutes. Season with salt and pepper and keep warm.
Cook pierogies in a large pot of lightly salted boiling water until tender, 12 to 15 minutes.
Transfer with a slotted spoon to skillet with onions. Toss gently to coat and serve immediately.
• Filling can be made 2 days ahead and chilled, covered.• Filled pierogies can be frozen 1 month. Freeze on a tray until firm, about 2 hours, then freeze in sealable plastic bags. Thaw before cooking.
