Heat a heavy bottom pot/large saucepan over medium heat. Add the butter and let it melt.
Add the diced onion, celery, carrots, pepper and garlic and sauté until softened, stirring occasionally. About 5-6 minutes.
Sprinkle the flour and mix well. Cook for about a minute, stirring frequently.
While stirring add the chicken broth, the cream and the beer. Mix well and add the seasonings - salt and pepper, mustard, Worcestershire and cayenne/red pepper flakes.
Let simmer for about 5 minutes and then remove from the stove. Take out the bay leaf.Use an immersion blender to puree the soup. Alternatively, transfer to a food processor or blender to puree but work in small batches to avoid the hot liquid blowing up.
Add the cheese to the pureed soup base working in batches and stirring. Place the pot back on the stove. Heat up over low heat to blend all the flavors, about 3-5 mins.
Serve right away, garnished with green onions and croutons (optional).