Using a mandoline or wide-blade box grater, shred the zucchini lengthwise into ribbons. Place ribbons on kitchen towels and salt the zucchini. Let drain 30 minutes then pat dry.
Line a 10x14-inch baking sheet with parchment paper, letting the edges hang over a couple of inches on each side. Scatter the zucchini on the baking sheet and toss with sliced onion and garlic. Arrange in even but not-too-organized layer and sprinkle with chili flakes.