1cupDubliner - shredded or substitue white cheddar
In a large saucepan melt 2 T butter over medium heat, add 1/4 cup Guinness Stout and the onions. Cook until completely caramelized and golden brown, about 1 hour, mixing every 10-15 minutes and adding water or Guinness to deglaze the pan as needed to prevent burning.
Add the garlic and thyme and cook until fragrant, about a minute.
Add the remaining 2 T of butter, let it melt, mix in the flour and let it cook for 2-4 minutes.
Add the stout and deglaze the pan.
Add the broth, Worcestershire sauce, grainy mustard, bay leaves, salt and pepper, bring to a boil, reduce the heat and simmer for 30 minutes.
Ladle the soup into oven-proof bowls on a baking sheet, top with the sliced bread and cheese and broil until cheese melts, about 1-3 minutes.