But that was not quite the full story.
Diana had instructed McGrady – formerly chef to the Queen – to prepare and serve her with a fat-free version of the mousse, while Oprah was served with the full-fat dish, loaded with mayonnaise, sour cream and a dollop of heavy cream as well.
“Diana never did tell her the truth,” McGrady liked to relate with delight.
Oprah has had her fair share of brushes with the royals, the most recent and memorable of which was her bombshell interview with Meghan Markle and Prince Harry.
In the two-hour special with the talk show queen, the renegade royals’ made a number of shock revelations which including clearing up rumours that Meghan had made Kate cry at her wedding as well as the Duchess having suicidal thoughts.
"I just didn't want to be alive anymore," Meghan told Oprah, before adding: "That was a very clear and real and frightening thought... I couldn't be left alone."
WATCH BELOW: Prince William responds to Meghan and Harry's explosive Oprah interview