Melt butter in 3 quart, heavy pan over low heat, preferably cast iron or copper. Add onions and cook, covered, 15 minutes. Uncover and continue to cook 30 to 40 minutes, stirring frequently, or until onions turn a deep golden color. Add stock and nutmeg. Cover and bring to boil. Reduce heat and simmer minutes.
Meanwhile, bring beer to boil in small pan over medium heat. Boil until reduced by half. Remove from heat, add cheese, stirring until melted. Transfer mixture to blender, add 1 cup onion soup, and blend until smooth. Reserve, remove onion mixture from heat and strain reserving liquid. Transfere onions to blender or food processor and puree until very smooth. Return puree to pan. Add reserved liquid and cheese mixture, add salt and season to taste with pepper. Serve piping hot in tureen. Makes 5 serving