This is the perfect quick, simple, hand held, comfort food!
Spread each piece of bread with butter on one side. Turn 3 pieces of bread over so that butter faces down.
Divide brie, apple, and caramelized onion into three portions
Stack one portion of brie, apple, and caramelized onion on top of bread, saving one piece of brie to put on top. Cover the stack with the other piece of bread, buttered side facing out. Repeat until you have 3 sandwiches.
Place onto a warm skillet or panini press. Cook until bread is warm and toasted, and cheese is melted. (You will need to flip sandwich once if using a skillet.) Repeat until all sandwiches are grilled!
Slice each sandwich in half and serve.
These sandwiches can be made in a skillet if you do not have a panini press. Just flip when you feel the bread is toasted enough on one side and allow it cook to on the other side.