DOUGH: In bowl, whisk flour with salt. In separate bowl, beat together egg, water and oil; stir into flour mixture to make soft but not sticky dough that holds together in ball. If necessary, add more water, 1 tbsp at a time, being careful not to make dough sticky.
Turn out onto lightly floured surface; knead just until smooth, about 5 minutes. Divide in half. Cover with plastic wrap or damp tea towel; let rest for 20 minutes.
MUSHROOM FILLING: Meanwhile, in heatproof bowl, cover dried mushrooms with boiling water. Let stand until softened, about 20 minutes. Drain, reserving liquid for another use. Chop mushrooms finely.
In skillet, melt butter over medium heat; cook soaked and fresh mushrooms and minced onion, stirring often, until tender and liquid has evaporated, about 8 minutes. Remove from heat; let cool slightly. Stir in dill, egg yolk, salt and pepper.
Working with half of the dough at a time and keeping remainder covered to prevent drying out, roll out on lightly floured surface to about 1/16-inch thickness. With 3-inch round cookie cutter, cut out rounds. Place 1 tsp of the filling on centre of each round. Lightly moisten edge of half of each round with water. Fold in half; pinch edges together to seal and crimp attractively. Place on tea towel-lined rimmed baking sheets; cover with damp towel to prevent drying out. Repeat with remaining dough and filling. (Make-ahead: Refrigerate for up to 8 hours. Or freeze on baking sheets, then transfer to freezer bags and press out air; freeze for up to 1 month. Do not thaw before cooking.)
In large pot of boiling salted water, cook perogies, in batches and stirring gently to prevent sticking together or to bottom of pot, until they float to top, about 2 minutes. With slotted spoon, transfer to colander to drain.
Meanwhile, in large skillet, melt butter over medium heat; cook sliced onion until golden, about 5 minutes. Add perogies; toss to coat and warm through. Serve with sour cream.
DOUGH: In bowl, whisk flour with salt. In separate bowl, beat together egg, water and oil; stir into flour mixture to make soft but not sticky dough that holds together in ball. If necessary, add more water, 1 tbsp at a time, being careful not to make dough sticky.
Turn out onto lightly floured surface; knead just until smooth, about 5 minutes. Divide in half. Cover with plastic wrap or damp tea towel; let rest for 20 minutes.
MUSHROOM FILLING: Meanwhile, in heatproof bowl, cover dried mushrooms with boiling water. Let stand until softened, about 20 minutes. Drain, reserving liquid for another use. Chop mushrooms finely.
In skillet, melt butter over medium heat; cook soaked and fresh mushrooms and minced onion, stirring often, until tender and liquid has evaporated, about 8 minutes. Remove from heat; let cool slightly. Stir in dill, egg yolk, salt and pepper.
