Dice the cucumber into small, even cubes about 1/4-inch in size.
Dice the tomato into pieces similar in size to the cucumber. Remove seeds if tomatoes are very juicy.
Finely slice the red onion. For a milder flavor, soak sliced onions in cold water for 10 minutes, then drain and pat dry.
In a large bowl, combine the cucumber, tomato, and red onion.
Add the lemon juice and salt, distributing evenly throughout the salad.
Add the chopped cilantro and any optional ingredients you're using.
Gently toss all ingredients to combine, being careful not to crush the vegetables.
Let the salad rest for 5 minutes to allow flavors to meld.
Taste and adjust seasoning if needed before serving immediately.