In a large frying pan over high heat, warm 2 tablespoons olive oil. Season tuna with salt and pepper. Add to the pan and sear over high heat, turning once, just until golden (about 3 minutes). Transfer to a plate and set aside.
Add remaining oil to pan and reduce heat to medium-high. Add onion and saute until softened, 3-4 minutes. Add garlic and saute 30 seconds. Stir in tomatoes, wine, red pepper, olives, and capers. Cover and simmer about 5 minutes to blend flavors. Stir in pine nuts.
Return tuna to the pan, spooning the sauce over it. Cook, occaisionaly spooning sauce back over tuna, until tuna is opaque throughout and the sauce is slightly reduced, 7-10 minutes.
Transfer tuna steaks and sauce to warmed plates and sprinkle with basil. Serve at once.