"Another old family recipe, handed down mother-to-daughter. I assume it is Irish, because it was from that side of the family, and it also makes a LOT! Fortunately, it is excellent reheated (and better the next day, too!) My mother always made this with leftover holiday ham, which makes it even more economical. Can be baked in the oven (very slow!) or cooked on the stovetop (my choice)."
1. Peel & slice potatoes, fairly thin if you will be baking the casserole; medium-thick if you will be cooking it on the stovetop.
2. Cut ham or spam into 1/2" x 1/2" chunks.
3. Begin layering into either a casserole dish or a large (dutch oven-style) pot. Start with a layer of potatoes, then scatter around a small handful of ham/spam, throw in a couple of small spoonfuls of margarine/butter, then sprinkle the whole layer with salt. Repeat layering process until the potatoes or ham/spam are used up (hopefully ending with potatoes.) Don't stress too much about the amount of salt; the potatoes will soak up most of it anyway. In fact, you may find you need more salt when it is served.
4. Pour milk into pot and cover. Use a little less milk if baking in the oven, a little more if cooking on the stovetop. (My Mom used to tell me, "You should just be able to see it coming up the side of the pot." but I found that that's always WAY too much.)
5. If baking, cook for about an hour at 375 degrees (check with a fork after 45 minutes.) If cooking on the stovetop, turn heat up until the milk begins to bubble, then turn down to medium-low (simmer) for about 20 minutes. Check potatoes with a fork. It should be able to penetrate with little effort.
Tip: I personally prefer to cook this on the stovetop, as it gets done a LOT faster, but it does have the disadvantage of possibly burning the potatoes onto the bottom if you turn the heat too high. Just don't try to scrape too hard on the bottom when you are serving it up!