Step 1: To prepare ponzu, place 1/2 cup orange juice, 1/2 cup lime juice, 1/4 cup grated onion, 1/4 cup low-sodium soy sauce, 1 tbsp chopped peeled fresh ginger 1 1/2 chipotle chiles in adobo sauce in a blender, and process until smooth. Step 2: To prepare salsa, combine 3/4 cup diced English cucumber, 1/2 cup diced plum tomato, 1/2 cup diced peeled avocado, 1/4 cup chopped fresh cilantro in a small bowl. Step 3: Sprinkle Tuna with salt. Heat a large nonstick grill pan over medium-high heat. Coat pan with cooking spray. Add fish; cook 3 minutes on each side or until desired degree of doneness. Cut each tuna steak diagonally across the grain into thin slices. Arrange sliced tuna over rice; top with salsa, and drizzle with ponzu.