1. Melt butter and oil in a medium sauté pan mover medium heat. Sauté sliced onion and pasilla pepper and for 5 minutes, reduce to medium-low and simmer for 15-20 minutes until onion is soft and translucent and somewhat jammy. In a large bowl combine chicken, oregano, salt and pepper and mix gently, mixture will be sticky so be careful not to overwork. Divide into 8 patties and refrigerate for a half an hour. Prepare grill. Meanwhile blend one pepper and ketchup in a small food processor.
2. Add another chile or more adobo sauce to taste for spiciness. Grill burgers for 15-20 minutes, flipping once, or until internal temperature is 160 degrees.
3. Add cheese slices. Warm buns on grill.
4. Serve with chipotle ketchup, onions and ketchup and arugula.