Combine Beef Base broth + charred onion purée in a large kettle.
Simmer 30–40 minutes to reduce by 25–35% until nappe consistency.
Add black pepper and vinegar to balance.
Hold hot for service.
Add ½ cup red wine vinegar, but taste as you go — you may need slightly less depending on onion sweetness.
Add 2–4 Tbsp brown sugar if needed to soften the charred edge.
Adjust salt only if necessary (Beef Base usually provides enough).