Cut the ends off the onions, and peel them. Then cut the onions into big round slices. It is important that you cut the onion so that the slices are equally thick, this way the onions will cook evenly. Heat the olive oil in a heavy bottomed pot over medium heat. Add the onions, and reduce the heat to low.

The onions will take 30-40 minutes to caramelize, you want the onions to reach a deep golden color. Stir them every few minutes so that they cook evenly.

Once the onions have cooked, let them cool. Slice the brie into then slices.

Assemble the sandwiches by first cutting the crust off the bread. Then spread mustard one the bread. Add a generous amount of roast beef, brie, and caramelized onions. Top with a second slice of bread. Slice the sandwich in half diagonally. Continue making as many sandwiches as you like.