In the bowl of a stand mixer, add light sour cream, lukewarm milk, egg, vegetable oil and ground sea salt. Using the dough hook attachment, process on medium-high speed until the ingredients are well blended. Add the flour and process on low speed until somewhat mixed.
Increase to speed 4 and continue until the dough gets together and clings to the hook, scraping the bowl with a spatula. If the dough is still sticky, add more flour, ½ tablespoon at a time. Knead the dough for 5 minutes on speed 5 until smooth
Form a ball by pulling the dough under and wrap it in cling film. Let it sit at room temperature for 1 to 2 hours or if not making pierogi the same day, transfer it to the fridge.
When it's time to roll, cut the dough in half and form two balls by pulling under. Wrap one with the same cling film while working with the other – the unused dough can be frozen for later use.
Flour the work surface generously and roll the dough thin, down to 1/8th of an inch thick. Using a 3 ½-inch cookie cutter, form circles – reuse the trimmings to make more pierogi.
Scoop up 1 ½ tablespoons of filling and place it in the center of each circle. Stretch the dough by starting right in the center and pinch it together. When the dough is sealed, continue by stretching, pressing and pinching going around until a half moon is formed – make sure the dough is well sealed and there are no air pockets. Crimp the edges and set aside while working with the others… At this stage, they can be frozen for later use.