Prepare the cucumbers: Dice the peeled and seeded cucumbers into pieces roughly the same size as the diced tomatoes.

Dice the tomatoes and jalapeño: Carefully remove the seeds and pulp from the tomatoes and finely dice. If using jalapeño, remove the seeds and ribs for less heat, then mince finely.

Combine the vegetables: In a medium mixing bowl, combine the diced cucumber, tomato, minced red onion, minced jalapeño, and chopped cilantro.

Dress the salsa: Pour the fresh lime juice over the vegetable mixture. Sprinkle with ground cumin, salt, and freshly ground black pepper.

Mix and rest: Gently toss all ingredients together until evenly coated with the lime juice and spices. For best flavor, cover and refrigerate for at least 15 minutes to allow the flavors to meld.

Serve chilled: Taste and adjust seasoning if necessary (more salt or lime juice). Serve immediately with tortilla chips, or as a topping for tacos or grilled protein.