Relax your lips, jaw, and tongue completely. Drop your jaw slightly and keep the tongue neutral.
How to pronounce onions in American English
Americans pronounce onions as UHN-yuhnz (/ˈʌnjənz/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "She sautéed the onions until they were golden brown and fragrant".
Now you try.
Record yourself saying "onions" and play it back. The mic stays on your device — nothing's uploaded.
Every sound in "onions".
2 syllables, 6 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Lift the middle of your tongue toward the roof of your mouth, but stop just short of touching. /j/ is an approximant, not a stop. The tongue tip stays down, lightly resting near the back of your bottom front teeth. Voice runs through the whole gesture, and the tongue glides smoothly down into the next vowel. The lips stay neutral or pre-shape for the upcoming vowel (rounding early for OO in <em>youth</em>, for example).

Relax your lips, jaw, and tongue completely. Drop your jaw slightly and keep the tongue neutral.
The schwa before N disappears — N becomes the vowel of the syllable. Go straight from the previous consonant to N.

Same position as S, but add vocal cord vibration. Feel the buzz.

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Common pronunciation mistakes in American English.
The textbook way isn't wrong — it's just not how anyone actually says it.
Inserting a vowel before the syllabic consonant.
In "onions", the short unstressed vowel before "n" disappears — the schwa is absorbed and the "n" becomes the syllable nucleus on its own. Schwa is absorbed — consonant becomes the syllable nucleus.
Stressing the wrong syllable.
Stress falls on the first syllable, not the others. Stretch UHN — keep everything else short and quick.
Pronouncing the unstressed syllable too fully.
Don't pronounce the first syllable too fully. The unstressed syllable reduces to a schwa — the lazy "uh" sound — in casual speech.

