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Combine cucumber, onions, cilantro, 4 1/2 tablespoons oil, 1 1/2 tablespoons vinegar, and chiles in processor. Using on/off turns, blend mixture until finely chopped. Transfer to bowl. Season with more vinegar, if desired, and salt and black pepper.
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Sprinkle fish fillets on both sides with salt, pepper, and cumin. Heat remaining 1 tablespoon oil in large nonstick skillet over medium-high heat. Add fish and cook 5 minutes. Turn over, cover, and cook until fish is just opaque in center, 4 to 5 minutes.
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Divide gazpacho sauce among 4 plates. Top each with 1 fish fillet. Scatter tomatoes atop and around fish and serve.