Bullion is a hunk of gold or silver, usually in a form suitable for trading or ready to be made into coins. Add an o to the word and move the i, and it becomes bouillon, which is a base for soup or stew.
Both words are French, but their relationship is a bit murky. It seems that both derive from the Latin word bullire, which means to boil. So does boil. Bullire also gives us bouillabaisse and boule, the French word for ball, though apparently ball is not directly related. If you’re familiar with the bullion stitch in embroidery, that comes from boule and has nothing to do with precious metals.
To classical Latin speakers, bulire also meant to make bubbles, which is sometimes a synonym for boil. Bubble is probably unrelated etymologically. It is a variation of the earlier burble, which is probably onomatopoeic.
Germanic languages gave us broth, also from a base word that meant boil and which also give us brew (more than 1,200 years ago).
Bouillon often is combined with cube; a bouillon cube contains dehydrated seasonings, vegetables and meat that are mixed with with boiling water to reconstitute bouillon.
The path of bouillon from boil or bubble seems obvious, but what of the blocks of metal? The etymology is a bit of a mystery, but it seems likely that the idea of boiling was applied to the process of melting metal. That’s a bit odd, since gold boils at 2,000 degrees Celsius but melts at about 1,000 degrees. Water boils at 100 C.
The word bullion first appears in English in relation to a place where precious metals were melted for coins. Maybe an early Anglo-Norman minter complained about the trading house as the furnace was going, “It’s boiling in here.” (That hyperbolic use of boiling is first attested in the Oxford English Dictionary in 1930.)
The mint use of bullion exists as late as the 18th century, according to the OED. Bullion was applied to a hunk of precious metal in the 14th century.
To differentiate the precious metal from the soup, you could think of the extra o as a round pot or soup bowl. And think of the French word for yes to remember the three vowels together: oui. If you have a mind for finance, you could remember that in a bull market, people are buying stocks rather than bullion.