HEAT the oil in a large, deep skillet or Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until it starts to brown lightly, about 5 minutes. Add the garlic and cook, stirring constantly, until it becomes fragrant, 30 seconds. Add the zucchini, turn the heat up to high, and cook, stirring occasionally, until the zucchini softens, 6 to 8 minutes. (You will need to add up to 1 cup of the reserved pasta water, a little at a time, as the zucchini cooks and becomes dry and sticks to the pan.)