Preheat an outdoor grill for medium-high heat. Combine ground sirloin, onion, grill seasoning, liquid smoke, Worcestershire sauce, garlic, adobo sauce, and chipotle pepper in a large bowl. Form the mixture into 6 patties. Season with salt and pepper. Place burgers on preheated grill and cook until no longer pink in the center. Place a slice of Cheddar cheese on top of each burger one minute before they are ready. Place burgers on buns to serve.