I made this last year between Thanksgiving and Christmas, when dinner ideas can be hard to come by.  It was within days of Thanksgiving actually, and so I also put the leftover stuffing (dressing) from the Thanksgiving turkey into the bottom of the pan first, spread a layer of mashed potatoes next, sprinkled some pieces of cut-up ham over the mashed potatoes along with diced up green, red and yellow onions.  Covered with tin foil, slapped it into an oven set at about 275 degrees for the few hours left in the afternoon (from about 2:30 until 5:30).  Just before we were ready to eat our meal, I sprinkled shredded cheese (a mixture of different kinds I had left in the fridge and wanted to use up) and left the tin foil off, stuck the pan back into the oven for about 10 minutes or so, and waa-laa - - supper in a flash!

https://sweetcsdesigns.com/cheesy-ham-and-mashed-potato-casserole/