The ornamental design for an adjustablegrommet and a grommet insert, as shown and described.
FIG. 1 is a front view of the adjustable grommet, the undisclosed opposite side being identical;
FIG. 2 is a side view thereof, the undisclosed opposite side being identical;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a reduced front view thereof, attached to the frame of an unclaimed racquet that is shown for illustrative purposes only and forms no part of the claimed design.