Wiktionary
all kidding aside
adv. (context speech act idiomatic English) (non-gloss definition: Used to attempt to make a serious point in a jocular conversation.) alt. (context speech act idiomatic English) (non-gloss definition: Used to attempt to make a serious point in a jocular conversation.)
Usage examples of "all kidding aside".
But you do know, don't you, all kidding aside, that I wouldn't think any less of you even if you were way off base here?