Fumet means aroma or flavouring. You can buy a truffle or mushroom fumet to add to a rich French sauce like sauce allemande, but the one you’ll see most often is fumet de poisson, the friend of the seafood cook, which enhances the flavour of the dish like nothing else can.
You can make it by gently simmering rigorously washed fish heads, spines, ribs, and tails of mild, non-oily, white-fleshed fish—making sure there are no traces of blood—with diced onions, leeks, garlic, celery, fennel, parsley, tarragon, bay leaves and sometimes carrots, along with white wine.
Boil for 20-30 minutes, enough time to extract the flavoursome gelatin from the bones. Beware that overcooking muddies the flavours. Then shove it all through a fine mesh strainer.
Or, like most people. you can buy the powder in the shop.
Image by Open Food Facts