Step 1Rub all sides of tuna steaks with sesame oil and sprinkle with salt. Cover and let stand while preparing cucumber salsa.
Step 2Combine onions, cucumber, grapefruit, cilantro, Thai pepper, garlic, lime juice, olive oil and 1/2 teaspoon salt. Let stand while grilling tuna.
Step 3Grill tuna steaks over high heat in stove top grill pan or on barbecue grill 5 minutes. Turn and cook until desired doneness, 2 to 5 minutes. Remove tuna to serving platter and serve Cucumber and Grapefruit salsa alongside.