I'm not sure what to think... I'm exhausted and have a headache.
But if you really want to quit you should do so; give notice and stick to it. If they don't find a replacement that is their fault and not your problem.
Another question or two. Did the nasty smell happen when the soup was fresh, or after being cooled and stored? What did you cook the soup and what did you store in? Cooking in cast iron can give off flavored as can storing in some plastics.
Nope, too much onion is quite unlikely to be the problem. Seek out a good recipe or two and follow them. Also, please don't succumb to the "arrogance of youth"... Tumors can happen at any age. There really could be a reason why you continually think you smell vomit.
Are you following a recipe or improvising? If the latter I'd suggest the former since your results are consistently unsuccessful. Soup should not taste like vomit. Perhaps you should also consider a medical exam... Brain tumors can cause phantom smells of feces and maybe vomit too.
I worked in Texas for a summer, at a remote Navy landing field. At the end of the day someone had to walk the runways and it was that persons responsibility to catch and dispatch all rattlesnakes found. On Friday the fire department ran a BBQ at which the rattler was the primary protein. Even though folks always say it tastes like chicken all it ever tasted to me was BBQ sauce and smoke.
... And in many cases the "Americanized" ethic restaurants are owned and operated by the immigrants themselves. They are willing to adapt and sacrifice authenticity to run a business that gives them the means to live in the US.