You don't necessarily have to use french bread. At our house, we always use plain sandwich bread. The onion and bacon are full of savoury goodness, making this dish unique.
I make this often at home. My husband and child love it.
If you don't have sandwich bread, you can of course, use french bread. If the soup is tepid, the bread will soak it up, so be sure to place the bread in hot soup, and place it in the oven immediately.
Enjoy Japanese food around the world.