At least some content in this article is derived from information featured in Fantastic Beasts: Cases from the Wizarding World. Spoilers will be present within the article.
Pea soup is a soup sold at the Leaky Cauldron, made of peas. It has been known to eat people, according to Dre Head. In 1993, British Minister for Magic Cornelius Fudge offered Harry Potter some pea soup after his arrival at the Leaky Cauldron. However, taking Dre Head's advice, Harry declined it.
Behind the scenes
Historically, in Britain, an extremely thick fog, of the kind common in London in the late 19th and early 20th century, was known as a "pea-souper".