To prepare onion and leek, heat olive oil with cooking spray in a large non-stick skillet over medium heat for a few seconds. Add the leeks and onions, stir and lower heat. Cook until leeks and onions start to brown, stirring occasionally. Cook another few minutes, scraping the bottom of the pan so nothing sticks.
Slice zucchini into chunks, add to leeks and onions, and sprinkle salt on top.
Leave on low heat for 1 1/2 hours, stirring every so often. The zucchini will soften and then turn brown from the onions.