In a 4-qt. Dutch oven cook bacon over medium heat until crisp. Drain on paper towels, reserving 3 Tbsp. drippings in pan. Add onion to reserved drippings; cook until onion is tender, stirring occasionally.
Stir in flour, mustard, and thyme (mixture will be thick). Add broth all at once. Cook and stir until bubbly. Stir in potatoes and beer. Bring to boiling; reduce heat. Simmer 5 minutes, stirring occasionally. Stir in cream; heat through.
Gradually add both cheeses, stirring after each addition until melted. Stir bacon and green onions into soup. If desired, serve with hot pepper sauce and additional bacon slices, cheese, and/or green onions.
