Raw zucchini, sliced into ribbons with a peeler, is mild-tasting; a short soak in vinegar and oil mellows and softens the onion but leaves enough bite to keep it interesting.
In a medium bowl, whisk together vinegar and oil; season with salt and pepper. Stir in onion, and let stand 15 minutes.
Meanwhile, using a vegetable peeler, slice zucchini into paper-thin ribbons, avoiding seeds. Add to bowl with dressing, season with salt and pepper; toss to combine.