Display Screen or Portion Thereof with a Graphical User Interface

The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Office upon request and payment of the necessary fee. FIG. 1 is a view of a display screen or portion thereof with a graphical user interface according to a first embodiment of our new design; and, FIG. 2 is a view of a display screen or portion thereof with a graphical user interface according to a second embodiment of our new design. The dashed broken lines depict the display screen or portion thereof with a graphical user interface and form no part of the claimed design.