Onion is not among the list of primary anti-diabetic herbs listed in Ayurveda and Traditional Chinese medicine.
Gymnema sylvestre, Trigonella foenum-graecum, Eugenia jambolana, and Curcuma longa are plants used in most Ayurveda based diabetes supplements. These plants have extensive scientific studies in support of their anti-diabetic properties.
Atractylodes macrocephala, Codonopsis pilosula, Dioscorea opposita, Flos lonicerae, and Pueraria lobata are used in Traditional Chinese medicine. These herbs have scientific literature supporting their ability to retard the progression of diabetes by reducing blood glucose levels.
Most scientific support for the above-listed plants is still restricted to animal trials. It is best to consult your physician before consuming herbal supplements containing the above-listed herbs. Anecdotal evidence does support their use as supplements. These, at best, have a role as an adjunct to modern medicine recommendations for keeping diabetes under check.
Popular literature is full of miracle cures of herbs, but my research in herbs on diabetes advises me to take these recommendations with caution. The diabetologist is your best friend for people with diabetes.
Reference: Natural Solutions for Diabetes