Wash and grate the zucchini. Crumble the feta, peel the onion and cut into small cubes. Halve and squeeze the lemon.
Cook the spaghetti in plenty of salted water according to the instructions on the packet.
Heat oil in a pan. Braise the onion. Add the zucchinis and cook for about 5 minutes. Then season to taste with lemon juice, salt and pepper. Stir in the feta.
Drain the spaghetti and add to the zucchinis in the pan.