FIG. 1 is an isometric view of mandolin slicer showing our new design. FIG. 2 is a left side elevation view thereof. FIG. 3 is a right side elevation view thereof. FIG. 4 is a front elevation view thereof. FIG. 5 is a rear elevation view thereof. FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof. The broken lines in the figures are for illustrative purposes only and form no part of the claimed design.