Preheat your oven to 350°F (180°C). Slice the baguette lengthwise, spread mustard on both sides, and place one slice of cheese on the bottom half.

Toast the bread in the oven until golden.

Chop the onion and sauté it in a skillet with butter.

Set the onions aside. In the same skillet, cook the two hamburger patties, then place a slice of cheese on each patty and cover to melt the cheese.

Take the baguette and assemble the sandwich. Place the two cheeseburger patties on the cheesy side of the bread, add the onions, and close the sandwich with the top half of the baguette.