Heat 1 tablespoon oil in heavy large skillet over medium heat. Add onion, celery, parsley and garlic. Sauté until onion is tender but not brown, about 10 minutes. Add tomatoes and bay leaf. Cover and cook until sauce thickens, stirring occasionally, about 15 minutes. (Can be prepared 1 day ahead. Cover and refrigerate. Rewarm before continuing.) Discard bay leaf. Mix in torn basil. Season sauce to taste with salt and pepper.
Sprinkle tuna with salt and pepper. Heat remaining 1 tablespoon oil in another heavy large skillet over medium-high heat. Add tuna and cook until golden and just opaque in center, about 3 minutes per side.
Transfer tuna to platter. Spoon sauce over tuna. Garnish with basil sprigs.