Sipperly laughed wheezily, then he saw that Janson was serious. An avaricious look crept over his fleshy features. “Well, joking aside, I’m really very fond of that dog,” he recovered. “He’s truly one-of-a-kind. Excellent guard dog … ”

Robert Ludlum, The Janson Directive, 2002.

joking apart/aside
said when you want to start speaking seriously about something after making jokes and laughing about it:
Joking apart, will you be able to manage on your own? [Cambridge Advanced Learner’s Dictionary & Thesaurus]

joking apart
seriously: said to recall a discussion to seriousness after there has been joking [CED]