2. While sweet potatoes are cooking, heat oil in a medium skillet over a medium-low flame. Add onions and cook, stirring regularly, until they are lightly golden, about 12 minutes. Remove from heat.
3. When potatoes are cool, peel them. Place them in a medium mixing bowl and mash them with a hand masher. Add onions, butter, salt, pepper and coriander and mix well. Set aside.
Make the dough:
1. Place flour on a large wooden board or in a large mixing bowl. Make a well in the flour and add beaten eggs and water. Gradually work the flour into the liquid, using first a fork and then your hands, until it is fully combined. Place the dough on a floured surface and knead until it is smooth and elastic, at least 5 minutes.
2. Divide the dough into 2 portions. Roll each out into a square no more than 1/8-inch thick. With a sharp knife or pizza cutter, cut the dough into 2 1/2-inch squares. Roll each square out a bit thinner before filling it.
Make the kreplach:
1. Set out a small bowl filled with one beaten egg. Place one teaspoon of filling in the center of a dough square. Dab your finger in the egg and then around the inner edges of the dough. Fold the dough over to form a triangle. Pinch dampened edges together and seal by pressing along edges with fork tines. Repeat with remaining kreplach. You will have some filling left over. (Kreplach can be frozen uncooked.)
2. Drop kreplach in a large pot of boiling salted water. Cook, stirring occasionally, until tender, between 20-25 minutes. Remove from pot, drain, and add to bowls of hot borscht topped with sour cream.