Preheat the oven to 400F. Line a large baking sheet with foil. Slice the bread in half lengthwise and put the bread (cut side up) onto the prepared baking sheet.
Melt the butter in a large skillet over medium heat. Add the onion and cook until softened and starting to caramelize, about 5 to 7 minutes, stirring occasionally. Add the garlic and cook 1 minute more, stirring constantly.
Stir together the onion/garlic mixture, cream cheese, and hot sauce in a large bowl. Stir in the chicken, salt, black pepper, mozzarella cheese, cheddar cheese, and blue cheese. Spread this mixture onto the bread.
Bake until the cheese is melted and the bread turns golden around the edges, about 12 to 15 minutes. Cool slightly, then top with scallion, slice the bread, and serve.
Notes
Hot Sauce: Do not use wing sauce here; just straight-up hot sauce is what we’re after. If you’re wondering about the heat level, ¼ cup of Frank’s RedHot Original Sauce is about “mild” heat level and ½ cup is about “medium”.
Make Ahead: Make the topping mixture up to 3 days ahead, and then top the bread and bake it as directed right before you want to serve.
The Even Quicker and Easier Version: Omit the butter, onion, and garlic. Use Frank’s RedHot Buffalo Wings Sauce instead of hot sauce and add 1 teaspoon onion powder and 1 teaspoon garlic powder.