Add onions and cook 20 minutes, stirring occasionally.
Add beef broth, water, sherry, Worcestershire sauce, pepper, and thyme to pot.
Increase heat and bring to boil, then reduce heat.
Cover and simmer for 5 minutes.
Divide into bowls.
Top with croutons, then mozzarella.
Broil until cheese melts and begins to brown.
The Chemistry of Allium Caramelization
Properly cooked onions require patience and steady low heat to break down harsh sulfur compounds into sweet, natural sugars without burning the delicate plant fibers. Keeping the butter on low heat during the initial twenty-minute sweat allows moisture to escape gradually while turning the sliced rounds translucent and uniformly golden. Although higher heat speeds up the process, it causes uneven scorching that introduces bitter notes to the broth.
Judging Doneness Visually
Broth is sufficiently simmered when the raw edge of the cooking sherry mellows into the beef broth after five minutes of covered bubbling. To gauge the final stage beneath the broiler, watch for the shredded mozzarella to blister and form light brown spots across the surface within two to three minutes. Monitoring the oven closely prevents the cheese from separating into an oily film while ensuring the croutons maintain a slight structural crunch under the dairy layer.
Adjusting Yields for Any Group
Scaling this preparation requires maintaining proportional ratios between the alliums and the liquid base, ensuring each portion receives an equal share of solids. Doubling the batch simply demands a wider saucepan to prevent overcrowding, which would otherwise trap steam and hinder the essential browning phase.