Stir the flour with salt in a medium/large bowl.

Warm the water with butter/oil in a small pot until very warm but not boiling.

Add the liquid to the bowl. Stir the dough with a spoon until roughly combined.

Knead the dough by hand or with a stand mixer fitted with a hook dough attachment, until it’s soft and smooth (it will take about 5 minutes with the mixer). If you follow the recipe exactly (especially if you weigh the ingredients, instead of using measuring cups), the dough should be perfect, but if for some reason it’s not, add some water if it’s too dry, or a little bit of flour if it’s too wet. If the dough already forms a ball, is relatively smooth, but still tough, it means that it’s not ready, it must be further kneaded until soft and elastic (after resting time it will be even softer).

Wrap the dough with plastic foil and leave to rest for 30 minutes.

Divide the dough into 2 parts.

Roll out the first part of the dough on a lightly floured surface (I like silicone rolling mats). Cut out rounds with pierogi cutter/biscuit cutter/a glass. The dough should be rolled out thinly. If the dough is hard to roll out, set it aside for about 5-10 minutes to rest.

Gather scraps, cover with plastic wrap and set aside.

Make the filling: In a medium bowl, stir the blueberries with sugar and flour/corn starch. If you have regular American large blueberries you will need less flour (or even none if they are not juicy) and if you have frozen blueberries or wild smaller blueberries (European blueberries) that are very juicy and break easily, you will need to add more flour. Make the filling directly before filling pierogi, don't leave the blueberries in sugar for too long or they will release juices.

Scoop some filling with a teaspoon and place on each round.

Fold the dough over the filling to create a half-moon shape. Press edges together, sealing and crimping with your fingers.

Place the pierogi apart on a towel lightly sprinkled with flour, cover loosely with a kitchen cloth so they don‘t dry out.

Repeat with the remaining dough.

Bring a large pot of salted water to a boil.

Cook the pierogi in batches (for a 8-inch (21 cm) pot I cook about 10-12 dumplings at a time). When they float to the water surface cook them for 1-2 minutes, then remove from the water with a slotted spoon. The cooking time will depend on the thickness of the dough.

Drain well and transfer onto a plate.

Enjoy!