My family has been eating this soup every Christmas Eve as long as I can remember. It’s a deceptively simple soup: The flavors are complex and rich, but it’s very easy to make, as long as you remember a few key steps. Over the years, my brothers and I have been slowly adapting this recipe as we become more experienced cooks. I think this might be my best version yet.
Chop the celery, carrots, onion, and half the bell pepper into uniform, half inch pieces. Melt the butter in a large soup pot over medium heat. When it's completely melted, and the foaming has subsided. Stir in the onion. Cook for about 2 minutes, until the onion is soft and golden. Add the celery, and cook until 2 minutes, then add the carrots, and the green peppers. Give the vegetables about 8-10 minutes to cook: you want them to begin to soften, and the butter to start to clarify a bit. Add a pinch of salt and stir well.
Begin to whisk in the flour. Add two tablespoons at a time, whisking the flour into the butter completely before adding more. Then begin to whisk in the broth, adding slowly, so the liquid is thick and the flour doesn't clump. Whisk in the Worcestershire sauce and the mustard.
Bring the liquid to a slow simmer, and let it simmer for about 10 minutes. Now you're going to whisk in the cheese. This is the trickiest part, because if you add it too quickly, the cheese will clump together. You want to add the cheese a handful at a time, whisking quickly. Wait until the cheese is completely melted in before adding more. Once all the cheese is whisked in and melted, let it cook for another minute or two. Then, add the beer. This will cause the soup to foam up a book. Just keep whisking until it subsides. Taste, and add salt if needed, and serve, with some crusty bread on the side.