OK, this is actually rather a small problem (no food poisoning or substances unfit for human consumption involved), but I wondered if anyone had any advice they could share.
I work in a small village, which has a little delicatessen 5 minutes walk from my office. A couple of times a week I'll go and buy my lunch there - they do large sandwiches of ciabatta or French loaf, amongst other things, with a variety of fillings and a wide selection of cheeses. I'm a fairly picky, vegetarian eater, so I only ever have one of the veggie pates, or mild or smoked cheese plain in my sandwich.
It's busy and popular, the staff are pleasant and obliging, and the food is delicious. The hygiene is also very good.
Now, as I said, I regularly go there for food. But almost every other time for the past few weeks, I've brought my sandwich back to the office, and been happily munching through, when I've had to stop immediately because there has been a strong taste of raw onion.

Which I detest with a passion. I can cope with it cooked and in a stew, curry, etc., and even if I have to "eat round it" in those situations (such as a curry with huge strings of it in), then it doesn't bother me overmuch.
But in a sandwich of white bread and cheese (especially the very mild cheddar which I like), the taste is incredibly strong, and (to me) repulsive.
I have eaten the mouthful with a grimace and then had to chuck the rest away. But the other day, (and apologies for potential TMI here) I actually had to spit the mouthful into my hand and throw it into the bin along with the small remainder of the sandwich (NB: Discreetly! My colleague in the office at the time didn't notice a thing).
I'm not allergic to onion, and there's no reason why I couldn't eat them if I liked them. I know this problem has arisen because of my own pickiness and strong distaste for this food (ironically, if I could occasionally taste a hint of meat in my food, I really wouldn't be bothered because the flavour's not an issue with me!)
I HATE wasting food, but I honestly can't choke through the other half of that long sandwich if I'm gagging on every bite (and I don't feel I could even offer the remainder to any of my colleagues, because after all, it's had my jaws round it!).
So...my question is, Should I go about saying something to the deli staff to see if they can prevent this happening in the future; and, if so, how?

I suppose it could have happened through the same knife being used to cut different cheeses, to lay slices on a sandwich that contains the dreaded onion, or lying on a board where there's some bits of salad - the staff's hygiene standards, as I said, are excellent, and you see them preparing the sandwiches, etc. in front of you. I've never seen them take a "grotty" (i.e. covered in soft, crumbly cheese/bits of Parma ham, etc.) knife to a different substance.
I don't want to sound as if I'm casting aspersions on their hygiene standards, complaining about a trivial matter, or to seem as if I'm "entitlement-obsessed". And it doesn't happen EVERY time, but has repeatedly on the odd occasion. But I want to be able to carry on going there, happy in the knowledge I'll be able to eat it ALL every time.
Should I say something? And what phrasing should I use?
Thanks to anybody who can help!
Edited because I CAN spell 'sandwich' [honest!]