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 the 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 horizontal jagged broken lines indicate a symbolic break in the display screen or portion thereof, and form no part of the claimed design. The appearance of any portion of the article between the break lines 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. Stippling in the drawings is used to indicate a contrast in appearance, with no specific color or other feature being claimed.