Peel the cucumber, if desired. Deseed the tomatoes, cucumber, and jalapeño. Finely dice the tomatoes, cucumber, onion, and jalapeño and add them to a large bowl. Then, finely chop the cilantro and add that to the mixing bowl. Squeeze in the lime juice and sprinkle in the kosher salt.
3 medium ripe tomatoes, 1 large cucumber, ½ medium white onion, 1 jalapeño, ½ cup fresh cilantro, 2 tablespoons (about 1 lime) lime juice, ½ teaspoon kosher salt