It's a control thing.
She KNOWS how dangerously allergic you are, it's nothing to do with forgetting. So forget all the 'ways to make her listen' stuff - this is about her, her stance is deliberate.
This is actually really common with people who want to be in control or are abusive in some way. They want to be the ones dictating your responses, is the best way I can think of putting it, and they really really get wound up by the thought of something to do with you - who are supposed to be in a subordinate position - having something about them that is an official 'You can't get your own way here'. It's in the same category as abusive husbands being furious with wives being ill, or getting more abusive when the wife is pregnant.
She hates that there is something about you that she has to listen to you over. Where what you say goes - and there is no way around it, she cannot override you or she will look very clearly like the control freak she is. Simply the fact that if she offered you fish you would refuse and that this response is 'cordoned off' from her being able to push what she wants - it's a red rag to a bull.
Limit contact loads, and start calling it what it is. Next time:
'No, I know you simply can't bear the fact that I've got an officially stamped excuse for you not getting to override me for once, but there's nothing I can do about it I'm afraid' (smile)
'Yep, still dangerously allergic, yep, I know that still makes you absolutely furious because you didn't get to decide it, yep, same old. (laugh)
She's a nasty cow, but you kind of know that.