Sorry but if anyone’s the douche it’s you. Calling someone a “Karen” is so wrong, on levels you clearly can’t comprehend. It’s reverse discrimination at its worst. Why is it okay for you to do whatever it is that you assume is fine, but when your roommate (who I assume must be white) expresses any opinion about it, she’s a “douche”? Discrimination doesn’t have a colour or a gender. Cooking preferences aren’t just based on race / culture. I’m a senior Caucasian and I cook a huge variety of dishes from every culture. Some things create a distinct odour that permeates the common spaces. Personally I think it’s admirable that your roommate tried to mitigate any cooking odour that might bother others. That’s what people are supposed to do in multi-family buildings.

Really? onions are number 1? I would imagine corn or soy for vegetables. US's biggest two crops anyway. And rice & pasta must be super popular too...
Anyway, cabbage & onion are the secrets to thrifty livin'. Maybe that's why she doesn't like it.

One person's perfume is another's skunk. Onion is a smell that gets into everything and stays there. It negates the results of doing laundry, and laundry is not cheap. I mean, really, those kind of odours fills a house, and not in a good way. Also, you sound like a huge pain when you should think about solutions instead of looking for accommodation over your offensiveness, which is totally rude.
Don't you use an exhaust fan? How about putting a curtain to cover the entrance so the odor in the kitchen stays there? Since you like to cook stinky foods, how about your getting an air purifier and blasting that when cooking so you don't offend your roommate?
You make the stink, you should fix the stink. Here's a tip: get lots of spider plants and a fan to circulate the air around them, these plants are famous for eating odours.
Between that, a kitchen entrance barrier and an air purifier harmony can be restored. Doing that is showing consideration.

the foundation of most dishes. I have an allergy to some onions which makes my eyes water and sting like crazy, but I still use them and put up with others using them because I know that they’re necessary and make things taste better. Move out or find a new roommate!!

I think you need to find a new roommate. You are clearly really badly suited and are going to drive one another crazy. The passive-aggressive thing can be particularly hard to take - I can empathize.

Otherwise, she sounds awful, but using "Karen" doesn't make you very sympathetic.

It's a stupid fad; let it go already.

... rather than seeing this through the bogus lens of critical theory, she just doesn't like the smell of onions. Not everyone enjoys the smell/taste of certain foods, this is called "personal preference." Also, sense of smell varies highly from person to person. Some people have almost no sense of smell, some people (like me) get on a bus and can smell nothing but a sea of disgusting shampoo/conditioner.

If you're going to write a bunch of insulting words about someone, get your spelling correct at least.
You sound like the bitch in this situation.

Be glad it isn't garlic.

You could skip the karening and the c word.