Know your onions
An astute time traveler would likely assume that know your onions along with derivatives such as know your apples, know your oats and others refers to someone knowing their business. Know your beans, or phrases similar, such as she knows her onions refer to someone who is capable and knowledgeable. Why onions became a popular version of the idiom in the 1920s is anybody’s guess, although the word onion was often used to refer to a person’s head during the era, but all versions of the phrase likely derived from the expression know your ropes.
Knowing the ropes was a nautical expression, originating in the days of sailing ships (which were still fairly common in the 1920s) when the sails which propelled the vessel were handled through the use of ropes (called lines) known as running rigging. Those lines which supported the masts when the ship was not underway were called standing rigging. An experienced sailor was expected to know what every line did and how they worked together, as well as know how to repair them if necessary. A sailor who had accumulated such knowledge was said to know his ropes. In the 1920s this expression was changed as to what knowledge was of, but they all meant that the person had knowledge of which they spoke.