Instructions

1. Make the dill oil: Place a bowl of ice water next to the stove. Bring a medium pot of water to a boil. Add the dill, boil for 30 seconds, then use a spider skimmer or slotted spoon to transfer to the ice bath. Drain, pat dry with paper towels, and transfer to a blender or food processor along with the oil and salt. Blend until smooth, about 3 minutes. (If the mixture seems too thick, add a little more oil.) 
2. Line a fine-mesh strainer with a coffee filter or a double layer of cheesecloth, then place it over a bowl. Pour in the dill mixture and set aside, stirring occasionally, until most of the liquid has drained out, 2–4 hours. (For quicker draining, drain directly through the sieve; some green flecks may remain.) 
3. Make the filling: Position a rack in the center of the oven and preheat to 400°F. Using a sharp knife, pierce each celery root all over (about 30 holes per root), then transfer to a rimmed baking sheet. Using your hands, rub the oil and salt evenly over both roots. Bake, basting with any juices every 20 minutes or so, until deep brown and soft, about 2 hours; set aside.  
4. Bring a generously salted pot of water to a boil. Add the potatoes and boil until tender when pierced with a fork, about 15 minutes. Drain and return the potatoes to the pot. Cut the celery root into 1-inch pieces and add to the pot with the potatoes. Mash until smooth, then stir in the twaróg, szafir, caraway seeds, and salt and black pepper to taste. 
5. Make the dough: In a large bowl, stir together the flour and salt. Using a wooden spoon, stir in the oil and 1 cup of warm water until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead until smooth and elastic, about 5 minutes. Invert a bowl over the dough and set aside for 15 minutes.
6. Divide the dough into three even pieces. Working with one piece at a time (covering the remaining pieces with a kitchen towel), roll the dough to a thickness of ⅛ inch, lifting the dough and dusting with more flour as needed to prevent sticking. Using a 2½-inch round pastry cutter or inverted glass, cut out as many rounds as possible, then gather the dough scraps into a ball. Repeat with the remaining two pieces of dough, adding the scraps as you go. (You should have about 40 rounds.) 
7. Place 1 tablespoon of filling in the center of one dough round, leaving a ¾-inch border. Fold the top edge over the filling to form a semicircle. Using the tines of a fork or your fingers, press the border to seal tightly. (If the edges don’t adhere, brush them lightly with water and press again; if there are any gaps, the pierogi may open during cooking.) Transfer the shaped pierogi to a lightly floured kitchen towel and cover with another towel to prevent drying. Repeat until all the pierogi are filled. 
8. Bring a large pot of salted water to a boil. Using a slotted spoon, gently lower 10–15 pierogi into the pot. Cook until they rise to the surface, 1–2 minutes, then use the spoon to transfer to a colander to drain. Repeat with the remaining pierogi. 
9. To serve, divide the pierogi among four plates. Top with a dollop of sour cream, drizzle with the dill oil, and sprinkle with caraway seeds.