If it's a meaning you want, 'sternly' means 'harshly' or 'firmly'.
You are so sternly.
I can give you several sentences.He spoke sternly to the misbehaving students.I backed down when he glared sternly at me."Follow your orders," the sergeant said sternly.
Example sentence - His father spoke sternly to him when he behaved badly.
Rigorous.
Firm or unyielding, uncompromising.
Lamented or sternly
They are synonyms, not antonyms.
Meaning hard, harsh, or severe in manner or character:
it means with sternness; in a severe manner.
Sternly disciplined, brave, undaunted
grimly, sternly, austerely, seriously, solemnly, thoughtfully
Look at them very, VERY sternly