Preheat the oven to 350°F (175°C).
In a mixing bowl, combine the shredded chicken and barbecue sauce until evenly coated; set aside.
In a small bowl, stir the melted butter and garlic salt together to make the garlic butter.
Using a sharp knife, slice the 12 Hawaiian rolls horizontally and place the bottoms in a baking dish in a single layer.
Spread the BBQ chicken evenly over the roll bottoms, then top with the thinly sliced red onion and the sliced cheddar cheese.
Fit the roll tops back on, cut between the sandwiches if desired, and brush the tops with the garlic butter, allowing some to run between rolls.
Bake for 20–30 minutes, until the cheese is melted and the tops are golden.
Remove from the oven, garnish with chopped cilantro if desired, and serve warm.