Bring water in a saucepan to a boil, add onion in a sieve and blanch for 1 minute. Remove and rinse under cold running water. Add zucchini to the pot with boiling water and cook for 30 seconds. Drain and put zucchini into a bowl with ice water. Move zucchini around so they cool evenly. Drain well.
Combine zucchini, mint, and basil in a large salad bowl. Mix oil, lemon juice, lemon zest, salt, and pepper in a small cup and pour over salad. Mix and refrigerate for 30 minutes.