Whisk lemon juice, olive oil, crushed garlic cloves and grated lemon peel in small bowl to blend. Stir in 2 tablespoons fresh basil and capers. Season vinaigrette to taste with salt and pepper. (Can be prepared 1 hour ahead. Let stand at room temperature.)
Prepare barbecue (medium-high heat) or preheat broiler. Season halibut steaks with salt and pepper. Brush fish with 1 tablespoon vinaigrette, dividing equally. Grill or broil halibut steaks until just cooked through, about 4 minutes per side. Transfer fish to plates. Rewhisk remaining vinaigrette; pour over fish. Garnish fish with remaining 1 table- spoon basil and serve.