Mushroom ketchup

The main UK brand is George (Geo) Watkins, and it is available online in the United States. If you don’t have it on hand, Worcestershire sauce or the water from soaking dried mushrooms can be used in its place. The ketchup has a dark-tasting, umami-rich flavor, so you need to replicate that. Chefs tend to use Maggi or other MSG-based flavor enhancers, and these are also a good substitute. Soy sauce would not be terrible.