Heat gas grill to medium-high. Combine cucumber, avocado, onion, lime juice, crushed red pepper and cilantro in a bowl. Season with salt.
Brush fish with oil and sprinkle with salt and pepper. Oil grill grates generously. Grill fish, turning once, until just cooked through, 6 to 8 minutes total. Serve with salsa on the side.
Find recipes with ingredients that you have on hand.