You might want to look into the Mastering Phoenix Framework book by @shankardevy… https://shankardevy.com/phoenix-book/
He goes through a TDD approach including iteration planning and user stories for all of the features and includes Hound (automated end-to-end browser testing) in addition to ExUnit testing. The resulting site you build is located here: GitHub - shankardevy/mango: Ecommerce site that you will build while learning Mastering Phoenix Framework · GitHub
I thought it was quite good!
