Marinate the chicken: In a bowl, mix diced chicken with sliced red onion, 2 tablespoons mayonnaise, and Montreal Chicken Seasoning. Cover and refrigerate for at least 4 hours.
Preheat the griddle to medium heat. 400 degrees
Cook the bacon until about halfway done. Push to one side of the griddle.
Add the marinated chicken and onions to the griddle next to the bacon. Cook for about 4 minutes, then flip and cook another 4 minutes, or until the chicken reaches 165°F.
Toss in the bacon to combine with the chicken and onions.
Pour BBQ sauce over the mixture and toss a few times to coat evenly.
Top with slices of Gouda cheese and cover with a dome to melt.
Place buns cut-side down directly on top of the cheesy chicken mixture. Once the cheese is melted, put your bun on top and slide a spatula underneath to flip (cheesesteak style).
Slice, serve, and enjoy hot off the griddle!