Mushroom soup reveals unidentified mushrooms name.

[JP_Finn]

If one cooks mushroom soup (^c, v, 4) from blanched/ boiled) mushrooms that the character has not identified: the game will still call the soup as the mushroom as it were known to the character.

E.g.
Herblore check on ‘peculiar mushroom’: not identified: peculiar mushrooms
Boil enough of the mushrooms to have 2lbs*. Do Herblore check: boiled peculiar mushrooms, edible.
Cook the mushrooms into soup. And the game, once cooking has started will prompt: “you set yellow finger mushroom soup to cook”. Any Herblore check will fail, similar to using the skill on non-plants.

*I’ve not tried to cook unidentified mushrooms before boiling.