You can have fructose malabsorption and be not sensitive to fructans and vice versa as far as I know. And no FODMAP table I've seen has ever indicated that onions are high in fructose.
-------------------- now: stable through EFI+FODMAP dieting (no lactose/no fructose/some fructans and some polyols)
before: IBS-D(pseudo-diarrhoea), bloating, often unbearable pain esp from too much fat: Apr 2007- Dec 2010