im on my 3rd large mountain biome i have more silver than i can use, stopped bothering to mine it now just tag it move on a thousand obsidian and more dragon eggs than the mind can comfortably deal with , but not an onion in sight!

i was under the impression they came from stone structures in the mountain , ive found a total of 2 in 4 biomes (one mountain area was small) a wooden one that did have a chest but no seeds and a stone tower with no chest (i was thorough in my search)

i had an absolute nightmare finding turnips to , im beginning to think theres a veggie conspiracy going on!

EDIT >>Lol not 5 minutes later hidden behind a big rock a tiny little wooden shack with a chest and 3 seeds! thanks you and goodbye mountains!