Display Screen or Portion Thereof with a Graphical User Interface

FIG. 1 is a front view of a first state of a display screen or portion thereof with a graphical user interface showing the claimed design; and,

FIG. 2 is a front view of a second state thereof.

The figures depict a transitional image sequentially transitioning between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design.

The outer dash-dot-dash broken lines illustrate a display screen that forms no part of the claimed design. The remaining broken lines are included for the purpose of illustrating portions of the display screen or portion thereof with a graphical user interface that form no part of the claimed design.