Heat 1 tablespoon of the oil in a large cast-iron skillet over medium-high. Add bell peppers, onions, and, if desired, jalapeño; cook, stirring often, until slightly softened and caramelized, about 5 minutes. Sprinkle with remaining 1/2 teaspoon salt. Add reserved marinade, and cook, stirring to scrape any loose brown bits from the bottom. Transfer bell pepper mixture to a platter, and cover with aluminum foil to keep warm. Wipe any extra juice from bottom of skillet, and return to medium-high.