Crossword Dictionary

austere, stern - a 
of a stern or strict bearing or demeanor; forbidding in aspect; "an austere expression"; "a stern face" 

stern, strict, exacting - a 
severe and unremitting in making demands; "an exacting instructor"; "a stern disciplinarian"; "strict standards" 

austere, severe, stark, stern - a 
severely simple; "a stark interior" 

Examples:

She gave the boy a stern look following his misbehavior.

Most employers wish to come across professional rather than stern, as a rigid personality can be unwelcoming to prospective employees.

My father was stern with his words when he told me that, so I know he was quite serious.