Onions are allowed, they have antifungal properties. Tomatoes on the other hand are an iffy one because they are technically a fruit. Each tomatoe contains about 3g of sugar as well! Even though this is not a lot it, it’d be best to save them for phase 2 if possible. Or use them sparingly whenever you absolutely have to have them.
Hope this helps. Onions contain about 5g of sugar per onion as well. I don’t think many of us eat a whole onion at a time though.
Here is some info: