Makes about 50 individual pierogi dumplings.
Make sure to use gloves when handling the raw nettle leaves. Blanche the nettle leaves in lightly salted boiling water for 90 seconds, then drain and rinse with cold water to stop cooking. Squeeze the nettle leaves between the palms of your hands, pressing them together over a sink to get all the extra water out of them (this is very important, we don't want the filling to be too wet!) Set the squeezed nettle aside.
Divide the filling into 50 equal-sized ovals, shape and size similar to a medjool date. If you have a kitchen scale this is easier, just measure the total weight of the filling in grams and divide that by 50. So for example, I ended up with 960 grams of filling total, which makes for about 20 grams per oval. Place them on a large baking sheet lined with wax or parchment paper, and refrigerate them uncovered while you make the dough.
Combine the flour and salt in a large bowl. Make a well in the middle and add the cracked eggs and the butter. Start stirring with a wooden spoon while slowly adding the water until incorporated. Turn out onto a well-floured surface and knead the dough until elastic and smooth, about 5 minutes. If the dough is too sticky you can add a touch more flour, but don't over-add otherwise the dough will be too firm.
Pick up one of the circle of dough and place a filling mound in the center. Fold the dough over it like a half moon, and pinch the edges together, pushing down on the filling with your finger as-needed to keep the filling inside the pierogi so it doesn't touch the edges. Crimp the edges by making folds as seen in this video: https://www.youtube.com/watch?v=EO5QnIVABdQ