though not an au gratin per se...
I make what my family has always called "Pork Blanket"...
no cheese involved...
grease a 9x13x3 pyrex pan with butter...
cut thin (1/4") slices of potatoes... and thin (1/4") onion slices...
place a layer of the potatoes and onions in the pan, dust top with flour and dot with small chunks of butter...
repeat a few times...
cover the top with 1/2" cut bone in center cut pork chops...
add milk to pan until it reaches just to the pork chops...
bake, covered with foil, at 350F for about 45 minutes...
remove foil and continue baking until the potatoes are tender...
very tasty...

you can also substitute leftover spiral cut ham slices for the pork chops...

NOTE... I like to par-cook the potatoes a bit before slicing, as I hate undercooked potatoes...