With a vegetable peeler or mandoline shave zucchini in thin strips. Salt lightly, then transfer to colander to drain, about 15 minutes. Meanwhile, shave carrots in strips and thinly slice onion.
Rinse zucchini and drain in colander. To assemble salad, in a 2-quart square dish layer one-third each of zucchini, lettuce carrots, and onion. For dressing, in a glass measuring cup whisk together lemon juice and oil. Drizzle some of the dressing on layered vegetables. Repeat layering and dressing twice. Cover and refrigerate 1 hour or up to 12 hours before serving.
To serve, sprinkle salad with fresh dill and pepper. Cut salad in rectangles with a sharp knife; lift out with a spatula.