Preheat your oven to 350°F. Lightly grease a medium glass baking dish to prevent the potatoes from sticking.
Peel the 4 Russet potatoes and slice them into thin, even rounds. For the most consistent cooking, try to keep the potato slices approximately ⅛-inch thick. Slice the Spam into thin pieces as well.
Arrange half of the potato slices evenly across the bottom of the prepared baking dish. Layer half of the sliced Spam over the potatoes.
Slowly pour 1 cup of milk evenly over the entire casserole. Pouring it gradually helps the milk work its way between the layers.
Cover the baking dish tightly with aluminum foil. Place it in the preheated oven and bake for 1 hour. Keeping the casserole covered helps trap steam so the potatoes can become tender.
Carefully remove the foil and return the casserole to the oven. Bake uncovered for another 20–30 minutes,
