1. Peel and trim onions. Halve lengthwise and place cut sides down. Thinly slice. Combine onion, vinegar, sugar, water, and cinnamon in container. Close and refrigerate at least 24 hours. Makes 1 quart spiced onions.
2. Line platter with lettuce. Spread tomato and cucumber slices over lettuce. Lift 2 cups spiced onions from marinade with slotted spoon and spoon over salad. Discard any cinnamon stick pieces. Sprinkle with mint and pepper.
3. Mix oil and lemon juice and drizzle over salad. (Remaining onion mix will keep for 1 week.)