Open source alternative to Node/Express for web app framework?

Hi all, in my current role we use Express as our web app framework of choice, which is great but I often feel that someone decided to run JS on the server as a cruel joke, asynchronicity is a pain regardless of how sweet the syntactic sugar is.

Anyway, do you know of any open source alternative web frameworks that are not Javascript based? I am considering Flask as our back end is Python based but does anyone actually use this?

Do you have any other suggestions? (Apart from PHP, I know Laravel is nice but we're not up for PHP).

Aside from not PHP other constraints would be, open source as stated, quick to prototype and iterate, ideally deployable to Heroku but not a deal breaker, choice of view engines would be nice and mature enough so that there are answers on stack overflow for the stumbling blocks we'll find when learning a new framework.

Any help or thoughts would be much appreciated, thanks in advance.