How to Reduce Hinge Mortise Depth

Shims.

Go to the local home center and get some of the (free?) laminate counter top samples in the kitchen cabinet section, they are about 3x5 inches. I then cut them to the same size as the hinge plate. Remove the hinge, place it on the sample and trace around it, it does not need to perfect but it should not be bigger then the hinge. I cut them with tin snips.

They are about 1/6 inch thick so sometimes you need to stack several of them behind the hinge.

Place enough of them behind the hinge to bring the hinge flush with the jam, hold the hinge over them and pre-drill, or mark them so you can pre-drill them, then install the hinge over them.

An alternative is to use a Dutchman. There are many YouTube videos on this. I like Tom Silva's tutorial from Ask This Old House.

Cut a plug of wood to the height and width of the two mortises. Use pine and cut the depth so the plug stands slightly proud when set in the mortises. You want the mortises to be a single mortise so chisel out any partition.

Cut the mortise to fit the plug. With a utility knife define the shape of the plug on the jamb. Cut away any parts that obstruct it's placement.

Wood glue the plug in place. Once dry plane down the plug to be flush with the jamb. Trace the new hinges outline on the jamb and cut the new mortise.