There are to places to find the list:
1. If you enter a project, you can click on the circular icon in the bottom-right corner - this will show all completed tasks from this project.
2. If you click on the Karma score in the top-right corner, you can go to "View All Completed" - this is a Premium feature which will display all tasks completed in all projects incl. each completion event of a recurring task and allow you to filter this list by project, time or label.