For the dough, mix 500 g flour, approx. 1 teaspoon salt, eggs and 100 ml warm water in a bowl, first with the dough hooks of the mixer, then with your hands to a firm dough. Place a tea towel over the bowl and let it rest for about 1 hour at room temperature.
For the filling, peel and finely chop the onion. Mix with minced onion and season with salt and pepper. Roll out the dough very thinly (1-2 mm) with a cake roll on a lightly floured work surface. Cut out 60 circles with a round cookie cutter (approx. 8 cm Ø) or a drinking glass.
Whisk the egg whites. Spread the edges of the dough with egg white all around. Put some chopping mass on each circle, fold it down to a semicircle and press the seams together firmly with your fingers. Spread the corners with egg white and press them together.
Cover and let it rest in the refrigerator for about 2 hours.
Pelmeni in portions in plenty of boiling salt water. Cook over low heat for 6-8 minutes. Lift one out and check if it is cooked. Then lift them all out and drain them well.
Wash the chives, shake dry and cut into small rolls. Melt butter in a pot. Swivel the pelmeni in it. Sprinkle chives over the pelmenis. Serve with sour cream.