Grate the courgettes on the coarse side of a grater. Salt generously and put them in a colander to drain for 20 minutes. Rinse thoroughly and put into a tea towel; squeeze out all the water by twisting the tea towel.
Soften the onion in one tablespoon of oil for about 10 minutes, stirring them around until they are tender but not brown. Let them cool and add them to the courgettes.