In a large non-stick saucepan, brown the onions in the butter until golden and soft, about 30 minutes. Season with salt and pepper. Dust with the flour and cook for 1 more minute. Add the beer and bring to a boil, stirring constantly. Add the broth and bring to a boil. Reduce the heat and simmer for about 10 minutes. Add more broth if necessary. Adjust the seasoning.
