Amanita caesaria is an edible mushroom, considered to be the favourite of the Roman emperors (hence its name). However, as Henry goes on to say, it resembles the highly poisonous Amanita phalloides, and is therefore not a popular choice among modern mushroom pickers.
Amanita phalloides has been implicated in the deaths of the Roman Emperor Claudius and Holy Roman Emperor Charles VI.
All of Julian's famous friends mentioned here tend to be English or American writers and who were prominent in the early or mid-twentieth century. It is particularly interesting that Tartt connects Julian with Harold Acton (1904-1994) as it creates an oblique reference to Brideshead Revisited by Evelyn Waugh and The Waste Land by T.S. Eliot. Acton was at Oxford with Evelyn Waugh. Although Waugh stated that Acton was not the primary inspiration for his character Anthony Blanche, he did inspire one particularly memorable scene in Brideshead when he proclaimed The Waste Land through a megaphone from a balcony. Brideshead is never directly mentioned in The Secret History, and yet the latter was clearly influenced by it.
Sole véronique is a famous French fish recipe that marries dover or lemon sole with tarragon, cream and grapes. See here for Delia Smith's version.