Prep: 5 minutes; Cook: 6 minutes. Using a grilling basket will help keep your fillets from falling apart. We like the Charcoal Companion's Platinum Rectangular Grilling Basket ($24.95, 888-789-0650).
Place garlic, salt, pepper, and cumin in a mixing bowl; mash to a paste with the back of a wooden spoon. Add yogurt and mint; stir well. Stir in cucumber gently, and set mixture aside.
Set up your grill for direct grilling, and preheat to high. Clean and oil grate (with vegetable oil).
Lightly brush olive oil on both sides of fish fillets. Arrange fish on grate or place in grilling basket (the pieces should run diagonal to the grate), and grill about 3 minutes per side or until cooked to taste, rotating each piece 90 degrees after 1 1/2 minutes to create a crosshatch of grill marks.
Spoon cucumber sauce over fish; garnish with mint. Serve immediately.