Display Screen or Portion Thereof with a Graphical User Interface

FIG. 1 is a front view of a first image in a sequence showing a display screen or portion thereof with a graphical user interface; FIG. 2 is a second image thereof; and, FIG. 3 is a third image thereof. The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which the images transition to another forms no part of the claimed design. The outer perimeter shown in broken lines in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the figures illustrate portions of the graphical user interface that form no part of the claimed design.