Feather HTTP https://github.com/featherhttp/framework. From the description: A lightweight low ceremony API for web services.
I didn't use it recently but Nancy was quite good: https://nancyfx.org/
Nancy is dead https://github.com/NancyFx/Nancy. Archived and stated that it is no longer maintained.
But the ASP.NET part feels bloated.
Is there a simpler .NET framework?