Some religions consider root vegetables like onions not okay to eat, because harvesting an onion involves killing the whole plant. 'Really holy' men and women might avoid any root vegetable to further their spiritual development.

"Some religions consider root vegetables like onions not okay to eat, because harvesting an onion involves killing the whole plant."

That is only one explanation for the taboo. In certain Indian and Nepalese religious strains, it is said that the devout cannot eat onions (or garlic) because of a folk belief that these vegetables encourage lasciviousness.

What is disgusting about tomatoes and onions? I can understand vitriol around meat, and I can imagine people who eat a lot of garlic being seen as smelly. But tomatoes and onions are healthy & fairly benign.

Speaking only from experience as a third party observer to my Indian friends & colleagues eating habits: onions are often lumped into the same category of 'unpure' ingredients along with garlic. Hindu tradition holds that such things cannot be offered to Gods, and negatively impact spiritual activities. So a non-trivial number of Hindus avoid these foods. Similar lines of thinking apply to other root vegetables. Jain dietary practices also tend to avoid garlic, onion, potoato, and other root veggies for reasons that I definitely do not fully understand.

Aversion to tomato is not something I have encountered though. I suppose it's possible I just haven't noticed it.

Kosher Jews say the same about shellfish and pork. Calling those sources of protein quote "bottom feeders" and therefor unpure.

I suspect that's a historical tradition made religious due to the fact that it's easy to get "poisonous" pork or shellfish that will kill you if not sourced or prepared correctly.

If your culture lives in a hot desert then not eating shell fish makes a lot of sense.

Pork was full of parasites until the advent of some relatively modern farming practices.