Mix the water, oil, matzo meal, onion and salt in a bowl and let cool for 10 minutes.
Whisk the eggs, add to the matzo meal and mix until it forms a smooth mixture.
Cover and put in the fridge for an hour to firm up.
Bring a pot of water to the boil and with wet hands, form the balls with a diameter of about 3½cm and cook about 7 kneidlach at a time, until they float and are well cooked, about 5 minutes. Remove from the pot with a slotted spoon.
Can be stored in a sealed container in the fridge for up to two days. Add to the soup before serving.