Add the snails, and then the mushrooms. Sprinkle with fresh parmesan cheese, salt, and pepper. The first time I made this recipe I made it with morel mushrooms. Their flavor overwhelmed even the garlic snails. It was good, but it didn't have the balance I was looking for.
Chanterelles seem to fit better.
If you're unable to find or afford chanterelles, you can substitute Chinese straw mushrooms, which are available in cans wherever Chinese groceries are sold. I guess the Asians wash them better before they dry them. Raclette cheese is so much better than any other kind of cheese in this recipe that it is worth looking for. What you really want to use is Normandy butter, but it's hard to get in North America. I don't know of any other states in which it is legal to buy raw butter.