Alliums, as are leeks, shallots and chives. Part of same bigger family as asparagus but probably not co-sensitivity.
The main issue with onion is the chemicals released from cut surfaces, which can trigger eye/nose reactions and potentially asthma. But there’s actually some evidence that onion has an anti-allergy action.
With garlic, there is a well recognised contact dermatitis relating to chopping it.
Otherwise, allergy is very rare. Potentially part of celery-spice-mugwort syndrome.
As with that syndrome, the problem for the allergic person is that not only is it not one of the 14 UK recognised allergens for food labelling and restaurants, but it can be included under “spices” if less than 2% of the overall product, without further detail.