To prepare the dough, measure the warm water and 3tablespoons olive oil into a large bowl. Sprinkle with sea salt, then whisk to combine. Add the flour, all at once, then stir to mix until the flour is blended in and you have a soft dough. Dust a countertop or a large cutting board with alittle flour, then dump the dough onto the flour. Knead for about 2-3 minutes, adding more flour alittle at a time (up to 3-4 tablespoons, depending on the moisture in the air and the dough) until the dough is soft and elastic but not sticky. Return to the bowl and cover with a kitchen towel for about 30 minutes while you make the filling and the topping.
To prepare the filling, place the partially peeled and quartered potatoes into a pot of lightly salted water, cover and bring to a boil over high heat. When it reaches a boil, remove the cover and boil for about 15 minutes or until tender when tested with a fork. Drain and return to the pot. Heat 2tablespoons of olive oil in a skillet over medium heat. Add 1 sliced onion and sprinkle with alittle sea salt. Cook for about 10 minutes, stirring occasionally until the onion is golden and soft. Add the onion and any juices to the drained potatoes. Using a potato masher or the back of a slotted spoon, mash the potatoes with the cooked onion slices until blended and roughly mashed but still alittle chunky. Taste for seasonings.
To prepare the topping, heat 2tablespoons of olive oil in a large skillet over medium heat. Add 1 sliced onion and sprinkle with alittle sea salt. Stir to coat with the oil. Cook for about 10 minutes, stirring occasionally until the onion is alittle golden and soft. Remove from heat and cover the skillet to keep warm.
Bring a large pot of lightly salted water to a boil over high heat.
On a lightly floured countertop or cutting board, turn out the dough. Dust the top with alittle flour, and lightly flour your rolling pin. Roll out the dough to about ⅛-inch thickness. Cut into 4-inch rounds, transferring the rounds to a flour-dusted cookie sheet. Continue cutting and re-rolling the scraps to cut more rounds until you’ve cut about 13 rounds.
One at a time, place a rounded tablespoonful of filling into the center of each round of dough, then lift the dough over to form a half circle. Lightly press the filling into the center of the half circle, and press the ends to seal. Continue until you’ve filled and formed all the pierogi. When the water reaches a boil, carefully drop the pierogi into the boiling water one at a time. Don’t stir them until they begin floating to the top, in about 3 minutes, then give them a gentle stir with a large slotted spoon. Continue cooking for a total of 5 minutes from the time you dropped the pierogi into the boiling water. Using a large slotted spoon, not a colander, lift out the pierogi one or two at a time, holding the slotted spoon over the pot for a few seconds to allow the water to drain back into the pot. Transfer the pierogi to a platter.
To serve, reheat the cooked onions. Spoon them with their liquids evenly over the pierogi. Serve with sour cream, if desired.