Boil the potatoes in a large pan for about 30 minutes, or till tender when pricked with a fork.
Cool potatoes slightly, then slice 1/4" thick.
In a large pan or skillet, melt butter over medium heat.
Stir in flour, cooking 1-2 minutes till golden.
Whisk in milk.
Bring to low boil over medium heat.
Remove from heat and season with salt and pepper.
Layer potatoes, ham and Swiss in buttered casserole dish. Repeat layers.
Pour white sauce over all.
Bake 350 degrees for 20 minutes.