Grate the cheeses and the zucchini with a cheese grater into a medium bowl. Add the eggs, onion, flour, and salt. Stir well enough to distribute ingredients evenly without mashing the zucchini.
Heat a medium frying pan to medium-high heat and add a small amount of oil. Drop zucchini mixture by heaping tablespoonfuls and cook for a few minutes on each side until golden.
