Grate the zucchinis and place them in a strainer for a couple minutes so it loses water. (note 1)
Grate the onion and mix with the chopped leek. Add the dry zucchini.
Add the egg, the flour, the cheese, the salt and a pinch of black pepper. Mix with your hands.
In a big pan heat enough oil to cover the zucchini fritters. Use about 1 inch of oil.
Fry the fritters with the help of two spoons (note 2).
Remove the excess oil using a rack or paper towel. Serve with mustard sauce or sour cream.
Notes
Zucchinis have a large amount of water. The best way to get rid of the excess of water is to squish with your hands and then let it sit in the strainer for a couple minutes, and then repeat the process. The dryer the zucchinis, the crispier the fritters will be.
Grab the mix with one spoon and with the other shape the fritters in the oil.