A few steps:
1. View > Hide Sidebar
2. Drag the split view divider between the message list and the conversation view all the way to the right edge of the window, collapsing the conversation view
3. Control-click the toolbar to customize it to your liking
4. Resize the window
Love the minimal view! It also wasn’t immediately obvious to me how to enable it. Was expecting to find it in the preferences or in one of the menu items rather than having to perform these steps.
If you move your mouse pointer up to the right edge of the window, from the left side, you should get a special one-way expand cursor that will allow you to reopen the collapsed viewing pane: