01 -
Melt butter in a large heavy pot over medium heat. Add finely chopped onion, celery and carrot; cook while stirring until softened, without allowing the butter or vegetables to blacken.
02 -
Add flour gradually and stir to coat the vegetables and form a pale, cohesive roux. Cook briefly while stirring the bottom and corners; lower heat if it catches.
03 -
Whisk or stir in the measured broth in stages, smoothing each addition before the next. Bring to a controlled simmer and stir until the base is evenly thickened and vegetables are tender.
04 -
Stir in mustard and beer. Simmer gently, then taste before adding salt and pepper, remembering the cheese will also add salt. This cooking stage does not guarantee alcohol removal.
05 -
Puree the base carefully. Use an immersion blender with its head submerged, or follow a countertop blender’s hot-liquid instructions and capacity limits. Work in batches if necessary.
06 -
Return the smooth base to very low heat or remove it from the burner if still very hot. Add grated Cheddar and Parmigiano-Reggiano in small additions, stirring until melted. Avoid active boiling during cheese incorporation.
07 -
Taste, adjust seasoning and serve with optional parsley, cheese, popcorn or croutons added to individual bowls. Refrigerate leftovers promptly in shallow containers; follow the separate reheating guidance above.