You need to use the pot of boiled mushrooms as an ingredient for another mushroom recipe - if you just eat them directly from the pot after boiling, they are still poisonous (because the water in the pot still contains the poison). So, for example, you can make a meat stew using the pot of boiled mushrooms as the mushroom ingredient - or boil the previously obtained mushrooms again to get mushroom soup. This does require cooking with two pots, so it's not immediately obvious.