はさておき / はさておいて

はさておき means "leaving X aside": you put one topic on hold and move on to the one you want to talk about. It goes straight after a noun, and the noun is what you set aside. はさておいて is the て-form of the same verb, さておく, and means the same.

Changing the subject

Two set phrases use it to steer a conversation. じょうだん冗談はさておき, "joking aside", turns from a joke to the serious point. それはさておき, "anyway", drops what was just said and opens a new topic.

はさておき or はともかく

はともかく also sets something aside, but to judge the rest: "X may be questionable, but Y is fine". はさておき postpones X and moves on, so it suits a change of topic.

Both belong to fairly formal speech and to writing. In casual talk, それはそうと and ところで do the work of それはさておき.