Fermented mushroom sauce sounds so much better than ketchup! Tell me more. Does it still exist commercially?
You can still sometimes find mushroom ketchup in UK supermarkets. It tastes a bit like Worcester sauce (spicy and 'brown' tasting), but milder as it has no anchovies in it.
> so much better than ketchup
Careful. What we refer to as "tomato ketchup" has been bowdlerized and degraded by being made shelf stable.
"When Every Ketchup But One Went Extinct" https://www.atlasobscura.com/articles/history-of-heinz-ketch...
Yes! Search for "mushroom ketchup", and you'll find various examples for sale. Whatever kinds I've had are nice on bread, and really nice with eggs, but I wouldn't want to eat with chips / fries.