Roast the red pepper directly over a gas flame or under a preheated broiler, turning occasionally, until charred all over. Transfer the pepper to a bowl, cover the bowl tightly with plastic wrap and let cool. When the pepper is cool enough to handle, discard the charred skin, seeds and stem. Cut the pepper into 1/2-inch pieces.
In a large skillet, toast the pine nuts over moderate heat, shaking the skillet a few times, until the nuts are golden brown and fragrant, about 2 minutes. Transfer the pine nuts to a plate.
Add the olive oil and diced onion to the skillet and cook over moderate heat until the onion is softened, about 5 minutes. Add the red pepper pieces and the chopped tomato and cook, stirring occasionally, until the tomato is softened, about 3 minutes. Transfer the vegetable mixture to a large serving bowl and let cool to room temperature.
Add the tuna and toasted pine nuts to the vegetable mixture in the bowl and gently toss to combine. Season with salt and pepper and serve.