In a small bowl combine the mustards and maple syrup. Briefly set aside. (And yes, you can use 3½ tablespoons of one type of mustard if you like/have that on hand.)
To assemble your sandwiches: Spread one side of each piece of bread with ½ tablespoon of maple mustard. Add a few slices of brie to each piece of bread. Top half of the slices with a layer of sliced apples, then add the roasted chicken. Close each sandwich with the remaining slices of bread with brie.
Spread both sides of each sandwich with a thin layer of softened butter.
Pre-heat a large sauté pan or skillet over medium heat. When hot add the sandwiches. (Depending on the size of your pan, you’ll most likely have to do this in two batches) Cover with a lid and cook for about 3 minutes until the underside is golden brown. Carefully flip the sandwiches, cover, and cook for another three minutes until the second side is golden brown and the cheese is completely melted.
Slice the sandwiches in half and serve immediately.