(informal) Synonym of know one's stuff, to be knowledgeable about a given subject or task.

Be highly knowledgeable about something.

"Know Your Onions" is the second episode of the seventh season of the American television series Agents of S.H.I.E.L.D. Based on the Marvel Comics organization S.H.I.E.L.D., it follows a Life Model Decoy of Phil Coulson and his team of S.H.I.E.L.D. agents as they race to stop the Chronicoms from unraveling history in 1931.