When I do "scalloped" potatoes, it's usually as a main dish to use up ham from a previous dinner. Leave the ham out if you want it as a side dish.
Take whatever baking vessel you're using (I never use a shallow dish for these) and fill it with whole potatoes. Not past the top, however. That's about how many potatoes you'll need. For every 3-4 large taters, use one large onion.
Slice all but one of the potatoes (in case you don't need it) and the onion about 1/8" thick. Spray or butter your baking dish. Start by layering potatoes in the bottom. If you've used about 1/4 of the potatoes, layer about 1/3 of the onion on top of them. The idea is to start and end with potato. On top of the onion, sprinkle about 1 heaping tbsp AP flour, salt/pepper to taste, a tiny bit of freshly-ground nutmeg, some leftover ham that's been cubed, and dot with butter (maybe 1-2 tbsp). Repeat process until the dish is about 1/2" from the top, with potatoes as the top layer. Pour about 1/2c milk in for every tbsp of flour you used, pouring it all over the top of the potatoes to let it dribble in. Top w/ some breadcrumb or grated cheese if you like, cover, and bake at 350F for about 1 hour. Have a look about 1/2 hour in to be sure the liquid is bubbling a bit. If it's a little dry, pour a little room temp (not cold!) chicken stock down one side, maybe 1/2c or so.
After an hour, the whole thing should be gently bubbling and making its own sauce. The potatoes should be done (poke down the center with the tip of a paring knife). Uncover and bake until the top is browned and the sauce a little thicker.
When you take this out, it's best to let it sit for 5 minutes or so to allow the sauce to set a bit. To serve, plunge down through the layers with a serving spoon like you would a souffle, and plate the scoop of goodness.
Sometimes I use a little granulated garlic for seasoning, and/or throw some parsley in there. Use what you like! I don't use the salt myself, but included it as most people do.
OK, now I want some! Too bad I made a big pot of spare ribs and sauerkraut yesterday, and the leftovers will be even better tonight!
Chuck