Whisk lemon juice, olive oil, garlic, and lemon zest in a small bowl. Stir in 2 tbsp fresh basil and capers, (reserving 1 tbsp basil for garnish)Season vinaigrette to taste with kosher salt and freshly ground black pepper.
Season 4-6oz halibut steaks with salt and pepper. Brush with 1 tablespoon vinaigrette divided equally among the steaks. Grill or broil until cooked through. Transfer fish to plates. Stir remaining vinaigrette and pour over fish. Garnish with remaining basil and serve.
Notes
This can be prepared 1 hour ahead. Cover and let stand at room temperature.