For dressing, in a screw-top jar, combine the salad oil, vinegar, garlic (if you like), salt, and pepper. Cover and shake well.
In a bowl, toss together zucchini and onions. Pour the dressing over the zucchini and onions; toss well. Cover and chill for 4 to 24 hours.
Serve in lettuce cups or atop torn mixed greens. Makes 4 to 6 side-dish servings.