In all seriousness and Simmer down

Simmer down and in all seriousness are semantically related in not humorously topic. In some cases you can use "Simmer down" instead an adverb phrase "In all seriousness".

Simmer down verb - Become quiet or calm, especially after a state of agitation.

In all seriousness and simmer down are semantically related in not humorously topic. You can use "In all seriousness" instead an adverb "Simmer down".