To set some expectations, we won’t have in-app authorisation to begin with.
When coming up with the list of things we needed to improve on for what turned into the blog post below, we decided to build 3D Secure 1.0, get it out there, and then iterate on it over time. We’ll be adding 3D Secure 2.0 support with in-app authorisation at a later date but to support 3D Secure 2.0, we also have to support 3D Secure 1.0 and fallback methods. 
The thinking behind this is that 3D Secure support is a big blocker for some people making all of their transactions with Monzo and was blocking a few major merchant acceptance issues. By building the minimum viable implementation, we unblock these issues and build a solid foundation for future work. We can build the minimal version on our backend with zero app work and get it out in a matter of weeks, rather than months. 
There’s also some exciting things that we want to do in the app that require seeding with merchant matching data from existing transactions. 
Rest assured that the user experience of even our initial minimal 3D Secure implementation won’t be super terrible, I’m making sure of that. 