Ancho Chili Powder will work instead of Chili Powder which has onions. Celery works in some recipes, so does just skipping it. For anything that absolutely needs something garlic/onion-y use ASAFOETIDA POWDER.
Any substitutes for onions? - Page 4
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I confess there was a time I thought the "onion sensitivity" was bogus, but now I know what it means. It wasn't easy to give up cooking with and eating onions but the bloating and all the other tell-tale effects are serious enough to convince me it is necessary.
Like others, I do enjoy spring onions, but only in moderation. Garlic does not seem to create a problem.
My biggest challenge is knowing what to eat at potluck events and when invited out for a meal.
In cooking at home, I'm having reasonable success using finely shredded parsnip to give a little extra zest in place of the onion. If I have extra shredded parsnip, I freeze it. I have also dehydrated it.
I tried cippolinis, but the reaction was the same as with eating onion.
Today I found this:
I will try sautéing onion and using the flavoured oil to enhance the hambone I've got simmering right now for dried bean soup.
You could try finding out whether your guest is allergic to raw or cooked onions, or just the common yellow onion. They may not be allergic to onions in total. If it's not just onions in total, there may be a different (non-alergic) reaction to the WHITE onion or even the RED onion which is the mildest.? Also I would consider chive heads, or maybe even onion powder? Blessings!