Heat olive oil in a pan over medium heat. Then add sliced onions. Let cook on low for 20-30 minutes or until caramelized.
Take pepper jack cheese and cut into small 1/2' cubes. Set aside.
Using ice cream scoop, scoop meet onto a sheet tray. Next, take your cubed cheese and place in the center of each piece of seasoned ground chicken. After you've placed all pieces of cheese in each scoop, roll meatballs into a circle with your hands until the cheese is completely covered.
Place meatballs on grill and brush with Stubb’s Original Bar-B-Q sauce. Continue to rotate and cook meatballs through. While the meatballs are grilling, chop cilantro for garnish.
Place buns on grill and lightly toast. Before plating meatballs, give them one last coating of Stubb’s Original Bar-B-Q sauce, then add to the bun.
Place caramelized onions and mozzarella cheeses slices on top. Cover grill for ~30 seconds to let the cheese melt.
Once cheese is melted, garnish with chopped cilantro. Serve!