For the pierogi dough, combine the flour and salt. Then add the olive oil and water and use a kitchen machine with a dough hook to make a smooth dough; or use a large spoon to combine, then use your hands to form a smooth dough. You might have to use a bit more water if it's too crumbly, or a bit more flour if it's too sticky. Dust the pierogi dough with flour, store it in an air-tight container and let it chill in the fridge for at least half an hour (you can also prepare it the day ahead and leave it in the fridge overnight).
For the filling, boil the potatoes until soft. Cook the onion in a bit of oil until soft and translucent (set aside half of the onion mixture to add later on). In a large pan, add the olive oil, boiled potatoes, onion, and add the salt, paprika powder, and cumin. Let it cook for about 5 minutes until toasty. Then mash with a potato masher or fork. Let the mixture cool off.
On a floured surface, roll out the pierogi dough until thin (but not see-through-thin). Cut out large circles and add about 1 tsp of filling onto one half of the circle. Fold over the other half, press the dough together along the filling. Repeat until you've used up all the dough.
Bring a pot with water to a boil, and toss in the dumplings. Let them cook for about 5 minutes until they float on top. Drain.
Heat up a large pan with olive oil, and pan-fry the drained dumplings until golden brown and crispy. Top them with the softened onion we've set aside earlier. For a nice green touch, also add rocket salad!