Mushroom ketchup was something I've been wanting to make for a long time. I love the fact that this was a common sauce so different from the ketchup we use today. In the early 1700s, ketchup was introduced to English explorers by the people of Singapore and Malaysia. Originally a sauce for fish, ketchup was made out of walnuts, oysters or mushrooms and was similar to soy sauce. The English expanded the use of the sauce and it became popular for fish and meat dishes.