How to Make It
In a colander, toss the cucumbers and squash with 1 teaspoon of salt and let drain for 10 minutes. Transfer the vegetables to a kitchen towel and gently squeeze them dry. In a large bowl, combine the vegetables with the scallions, jalapeño, cilantro, mint, lime juice, lime zest and yogurt; stir well.
In a small skillet, cook the garlic in the 2 tablespoons of oil over moderate heat until fragrant and lightly golden, about 1 minute. Add the cumin, mustard and black onion seeds and shake the pan until the seeds begin to sputter and pop. Stir the seed mixture into the vegetables and season with salt and pepper.
Rub the fish lightly with oil and season with salt and pepper. Grill or sear the fish in a preheated grill pan over moderately high heat, turning once, until golden and cooked through, 6 to 7 minutes. Transfer the fillets to plates, spoon the vegetables alongside and serve.