Place 1/4 of the dry ranch mix in a small bowl and mix in the mashed avocado.
In a medium bowl, combine the remaining ranch mix, ground beef, onion, and barbecue sauce. mix well.
Form the mixture into 4 large flat patties. Preheat an outdoor grill to medium heat. Place the patties onto the grill and grill over medium heat for 5 to 6 minutes per side or until cooked through, the internal temperature has reached 160 degrees F.
During the last few minutes of cooking top each patty with about 3 tablespoons shredded cheese and grill until melted.
Split the hamburger buns and place them cut-side down onto the grill and lightly toast.
To assemble, top each hamburger bun with lettuce, tomato, a grilled hamburger patty and some of the seasoned avocado.