I had the same issue with the onions in the mountains. I got every other resource in abundance yesterday foraying into the mountains, crafted, and then upgraded, all of my gear and not a single onion seed.
Finally, just before I went to bed I found one more log cabin on my way back to base camp, and finally got the onion seeds.
Kind of wacky that seeds end up being the rare-hard-to-find upgrade materials in the game, but I guess that's what it is.

I also finaly got some acorns, cutting down an oak tree near the base camp, just before logging off. I've wanted those for like....100 hours now in this save. Of all the upgrades and new gear I got, those acorns were still the highlight of my game yesterday.