Line a baking tray with aluminum foil and place the croissants on the tray.
Use a serrated knife to cut the croissants down the middle (from the top), almost all the way through.
Open up each croissant and spread some of the mustard on one side, and some of the mayonnaise on the other.
Place a cooked hot dog wiener in each croissant.
Wedge in a couple slices of Brie cheese on one side of the wiener, and caramelized onions on the other side.
Sprinkle some grated mozzarella cheese on top of each prepared hot dog.
Turn on the oven broiler and make sure there is a rack in the middle position. Place the baking tray on that middle rack, so that the cheese can melt slowly, without burning.
Remove the tray from the oven when your desired golden color has been reached and serve right away.