Display Screen or Portion Thereof with a Graphical User Interface

The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing our new design.

The outermost rectangular broken line in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines represent elements of the graphical user interface that form no part of the claimed design. In embodiments of transitional graphical user interfaces, the appearance of the transitional image sequentially transitions between the images shown. The process or period in which one image transitions to another image forms no part of the claimed design.