How to Make It
In a medium saucepan, combine 1/2 cup of the sugar with the wine and bring to a boil, stirring to dissolve the sugar. Add the red onion, cover and cook over high heat, stirring occasionally, until softened, about 5 minutes. Drain the onion slices and discard the liquid. Return the onion to the pan. Add the remaining 1/2 cup of sugar and the red wine vinegar and cook over moderate heat, stirring occasionally, until syrupy, about 6 minutes longer.
Meanwhile, in a large skillet, heat 2 tablespoons of the olive oil until shimmering. Add the Spanish onion and cook over moderately high heat, stirring, until softened, about 5 minutes. Add the sliced garlic and roasted red peppers and cook over moderate heat until the garlic is softened, about 5 minutes. Add the sherry vinegar. Scrape the mixture into a food processor and puree until fairly smooth. Season the red pepper sauce with salt and pepper and transfer to a bowl. Rinse out the skillet and pat dry.
Heat the skillet until hot but not smoking. In a small bowl, combine the remaining 1/4 cup of olive oil with the minced garlic, cayenne and cumin. Rub the mixture over the steaks and season with salt. Add the steaks to the skillet and cook them over high heat, turning once, until they are medium-rare, about 5 minutes. Transfer the steaks to plates and serve with the onion marmalade and red pepper sauce.