In a 12-inch skillet, heat 1 1/2 tablespoons oil over medium-high heat. Season tuna generously with salt and pepper. Arrange half of tuna in skillet in a single layer to prevent crowding. Cook, turning once, until golden brown on top and bottom, about 1 1/2 minutes per side (3 minutes total). Tuna should be slightly pink inside. Transfer to a platter. Cover to keep warm. Add another 1 1/2 tablespoons oil, and repeat with second batch.
Wipe out the bottom of the skillet. Add remaining 2 tablespoons oil and the onion. Cook over medium heat, stirring, until onion is golden brown and soft, 4 to 6 minutes. Add tomatoes and wine, and simmer, stirring often, until the tomatoes' skins just begin to wrinkle, about 2 minutes. Cook at a bare simmer until sauce thickens slightly, about 5 minutes more. Add basil to skillet, and cook, stirring, until wilted. Season with salt and pepper. Spoon hot sauce over tuna just before serving.