A large non-stick pan with cooking spray. Add onions; cook on very low heat 2 hours or until onions are golden brown, stirring frequently and scraping the bottom of the pan. If onions are not browning evenly, add a little water to the pan.
For each serving:
1 bread slice with 2 cheese slices, 1 Tbsp. (15 mL) caramelized onions, 1/4 cup (50 mL) chorizo, 1/3 cup (75 mL) arugula, 2 additional cheese slices and second bread slice.
Non-stick pan or griddle with cooking spray. Cook sandwich on medium heat 3 to 5 min. or until cheese is melted and bread is golden brown on both sides.