I found a simple solution, in my layout I added:
- if current_page.data.title? title= current_page.data.title + " | My Website" - else title My Website
When in your slim pages you have :
--- title: My page title ---
You end up in your generated code using middleman with the title
<title>My page title | My Website</title>
And if the page has no title it will default to :
This was found when trying to improve diesel documentation.
I hope this can be useful to somebody else.