Position an oven rack in the top third of the oven and preheat the broiler.
Put the red peppers on a baking sheet. Drizzle with a little oil and rub to coat the peppers lightly; season with salt and black pepper. Broil skin-side up until blackened. Transfer to a bowl, cover and let steam until cool, at least 15 minutes. Discard the skins and cut into chunks.
Heat the oil in a large skillet over medium heat. Add the onion and cook until soft, about 8 minutes. Remove from the heat; add the peppers, raisins, capers and vinegar and toss to combine. Season with salt and black pepper. Toss with the arugula and serve.
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