In a medium skillet over medium heat cooked the sliced onions in a tablespoon of butter until caramelized. Set aside.
Heat a clean skillet over medium heat.
Butter one side of each piece of bread and set aside. Butter the entire side, edge to edge.
Lay one slice of bread, butter side down, in preheated skillet. Top with cheese, caramelized onions, roast beef and more cheese. Top with bread slice, butter side up. Repeat with remaining ingredients. Turn when buttered side is golden and cheese is melting. Don't fiddle with the heat. Remove when both sides are golden and cheese is melted.
Slice sandwiches in half and serve.
Notes
To remove the rind from the Brie: Cut the rind off with a very sharp knife or peel it with a vegetable peeler around the edge.