How long does it take to caramelize the onions?
Plan 45–60 minutes of patient cooking over medium-low heat, stirring frequently. Rushing this step flattens the flavor; proper caramelization is non-negotiable.
Can I make this ahead?
Yes. Cook the soup fully, cool, and refrigerate for up to 3 days. Reheat gently on the stovetop, then assemble with baguette and cheese under the broiler just before serving.
What if I don't have cognac?
Substitute dry white wine or brandy. Cognac adds a subtle grape note, but omitting it entirely still yields excellent soup if you prefer.
Why use Gruyère instead of Swiss or cheddar?
Gruyère has a nutty, slightly sweet flavor that complements caramelized onions better than milder Swiss or sharper cheddar, and it melts smoothly without separating.
Can I scale this recipe?
Yes. All ingredients scale proportionally. The caramelization time may increase slightly with larger batches; stir frequently and taste for doneness.