Yes - they contain fructose but they don't contain more fructose than glucose so the fructose is not a problem.
According to the Barrett reference you provided it say "Thus, fructose is well absorbed in the presence of equimolar glucose in the proximal small intestine (25), whereas free fructose is slowly absorbed and such absorption occurs right along the length of the small intestine." They go on to say "since fructose is absorbed efficiently in the presence of an equimolar concentration with glucose, people with fructose malabsorption need to avoid foods high in free fructose, but can manage those with balanced concentrations of fructose and glucose (or a greater concentration of glucose)."
Onions fall into the latter category. It is the fructans that can be a problem for some but not the free fructose.
It would be helpful if you noted in your signature that you don't have IBS. As far as I known almost everyone in this forum has IBS.
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