The Dining of Aristocrats is a book that can be found in the Witch's House on the table in the Kitchen, which serves as a clue of what to do next.
The Dining of Aristocrats
"People of ruling and wealthy classes have long enjoyed dining with silverware. By using such hard-to-clean cutlery, they could hire servants to demonstrate their wealth. In addition, silver utensils change color upon contact with poison, making them quite useful."
The book hints that you have to use the Silver Key given by The Chef on the bowl of poison in order to proceed.