In Pentos Illyrio
served Tyrion Lannister
a dish of mushrooms kissed with garlic and bathed in butter - art by Tiziano Baracchi. © Fantasy Flight Games
Mushrooms are similar to our world’s mushrooms. As in our world some varieties are edible while others are extremely poisonous. Mushrooms are used in various food dishes in Westeros and Essos.
Death by mushroom
Magister Ordello was poisoned by a mushroom. It is said that pain is not so much. Only some cramping in the gut and a sudden ache behind the eyes, and then it is done.
A Dance with Dragons
While in Illyrio's manse Tyrion Lannister picks seven poisonous mushrooms from the garden and stores them in one of his boots.
Later on Illyrio serves Tyrion a dish of mushrooms kissed with garlic and bathed in butter. He tells Tyrion the taste is exquisite. Tyrion grows suspicious, thinking they are poisonous although in reality they are not. Illyrio uses Tyrion's suspicions to his advantage to convince him to travel to Daenerys Targaryen. Tyrion tells him he has no wish to die, but can't think of anything he has left to live for. Illyrio tells him he has "nothing", but they can change that. He then plucks a mushroom and eats it, declaring it delicious.
Grey mushrooms spot from the Three-eyed crow's brow and cheeks. Bran notes that Under the hill a hundred kinds of mushrooms grow.
||Better a mushroom than a sword through your neck, is it not so? Why die with the taste of blood in your mouth when it could be butter and garlic? 
- Illyrio Mopatis
, to Tyrion Lannister
||My sister has mistaken me for a mushroom. She keeps me in the dark and feeds me shit.
- Tyrion Lannister
, while he is lying incapacitated after the Battle of the Blackwater