Display screen or portion thereof with a graphical user interface

[0001]The single FIGURE is a front view of a display screen or portion thereof with a graphical user interface according to the new design.

[0002]The outermost rectangular broken line in the FIGURE illustrates a display screen or portion thereof and forms no part of the claimed design. The dash-dot lines in the FIGURE indicate boundaries of the claimed design and form no part thereof. The remaining broken lines represent elements of the graphical user interface that form no part of the claimed design. Stippling in the drawing is used to indicate a contrast in appearance and forms no part of the claimed design.