Dice cucumber, tomato, and red onion into uniform ¼-inch cubes. Soak the diced onion in cold water for 5 minutes to mellow sharpness, then drain completely.
1 medium cucumber, 2 medium tomato, ½ medium red onion
Whisk fresh lemon juice, olive oil, dried mint, salt, and black pepper together in a large mixing bowl until the oil and lemon juice emulsify slightly.
2 tablespoons lemon juice, 2 tablespoons olive oil, 1 teaspoon dried mint, ½ teaspoon salt, ¼ teaspoon black pepper
Add the diced cucumber, tomato, and drained onion to the dressing. Toss gently with a large spoon until all vegetables are coated evenly.
1 medium cucumber, 2 medium tomato, ½ medium red onion, 2 tablespoons lemon juice, 2 tablespoons olive oil, 1 teaspoon dried mint, ½ teaspoon salt, ¼ teaspoon black pepper
Cover the bowl and refrigerate for 30 minutes to allow flavors to meld and onion sharpness to soften.
Before serving, add fresh parsley or mint if using, sprinkle sumac if desired, and taste to adjust salt or lemon juice.
¼ teaspoon sumac, 2 tablespoons fresh parsley or mint, ½ teaspoon salt, 2 tablespoons lemon juice