1. In all seriousness. Here's an example from a physics textbook.

2. You can't in all seriousness think they'll give you the job!

3. In all seriousness,(Sentencedict.com ) there is nothing else I can do.

4. ‘Bestness,’ Nancy said in all seriousness.” I loved Timothy Featherstone, the dreadful No

5. 'Playing with Richie was the highlight of my musical career,' said Sonny in all seriousness.

6. A while ago, I was on a call-in radio program, and one of the callers asked about the words “commentate” and “Conversate.” She hadn’t even thought “Conversate” was a word until a friend of hers used it, and she laughed, thinking it was a joke, only to realize her friend was using it in all seriousness.