Dice onion finely. Bring a pan to medium-high heat, add oil, and fry the onion pieces until they are translucent (do not let it brown). Take off heat and let them cool slightly.

Whisk the eggs, until the yolks and whites are well combined. I believe that the more you whisk, the fluffier will your kneidlach be (this is probably not true, but do whisk the eggs a bit). Fold the eggs into the matzo meal. Add the onions and fold everything together. Add salt and fold.

Boil a cup of water. Then slowly add water to kneidlach. Start by adding ¼ of a cup and mixing to combine. If mixture is still dry, add another ¼ cup. Mixture should be sticky at this point, enough to form it into balls of similar size to meatballs. Let mixture rest for 30 minutes. Test stickiness level again, and if it forms into balls that stay together, then proceed.

Bring a large pot of salted water to a boil. With clean hands or with two spoons, form the kneidlach mixture into meatball-sized balls, and drop them one by one into the boiling water. Let kneidlach cook on medium-high heat for 10 minutes, until water is boiling again and the kneidlach rise to the top (do not remove them as soon as they float to the top, but give them some time to fluff up). Remove them into a large container or bowl with a slotted spoon.

When ready to eat, add two or three kneidlach balls to each bowl of soup. Garnish with cilantro or parsley, and dig in.