Add the onions and sauté until starting to caramelize, about 10 minutes (make sure the heat isn’t too high or the onions will burn).
Add the olive oil and mushrooms and cook until dark and soft, about 10 additional minutes.
Add the broth, stout, black pepper, and thyme, simmer for ten minutes.
Preheat the broiler.
Ladle the soup into 4 oven safe bowls.
Top with bread, then cheese.
Place under the broiler until cheese has melted.
Notes
young cheddar melts better than aged cheddar, don't be afraid to save the expensive stuff for something else and use the younger, cheaper cheese for this soup.