Ingredients :

• The onions — Take classic yellow onions, not red ones — they release too much sugar and turn burgundy during cooking. You need a lot of them, six large onions for four people. They will reduce by half during caramelization, which is normal.
• The mushrooms — Button mushrooms are enough. If you want to go further, mix them with brown mushrooms or shiitakes — the taste is much more pronounced. Don’t slice them too thin, otherwise they disappear completely into the soup.
• The broth — Beef or vegetable broth, it’s up to you. Use a decent quality broth — cubes work, but avoid very salty brands. The soup reduces a bit during cooking and the salt concentrates with it.
• The croissants — One plain bakery croissant per person, fresh or from the day before. Not those industrial vacuum-packed mini-croissants — they have neither the texture nor the flavor required. It doesn’t need to be AOP butter, it’s going to melt into the broth anyway.