Thinly slice the onion and mushrooms. Heat some oil in a frying pan and gently sauté the onion for 5 minutes, then add the mushrooms and herbs. Put a lid on and sauté for 5 minutes over low heat.
Add the cheese and turn off the heat. Let it cool a bit.
Turn on the waffle maker.
Cut a lid off the croissants (lengthwise). Scoop or tear out half of the soft inside of the base.
Fill the bases of the croissants with the cheesy mushroom, then lay a thin slice of roast beef on top (you can spread a little mustard or horseradish onto the beef, if liked.)
Press the top half on top of the stuffed base, then put one croissant in the waffle maker (or both if they fit) and press the top part of the machine down.
Cook for 2-3 minutes or until crispy. Wipe the iron plates of the waffle maker and cook the other croissant.
Serve warm with a salad on the side.