The fresh taste and refreshing crunch of cucumber makes this a perfect addition to any grilled fish dish.
Heat a small skillet over medium heat. Add cumin seeds; toast, stirring, until they are lightly browned. Transfer to a small bowl; let cool.
Peel and seed the cucumbers, and cut into 1/4-inch dice. Place in a large bowl, and add celery, scallions, poblano chiles, cilantro, lemon juice, and toasted cumin seeds. Season with salt, and serve immediately.
Relish can be made up to one day ahead and stored in refrigerator; add the salt just before serving.