Step 2: Slice the Potatoes
Scrub or peel the potatoes and slice them into thin, even rounds approximately one-eighth to one-quarter inch thick. Uniformity of thickness is the single most important variable in this recipe — slices that are too thick will remain firm in the center when the rest of the dish is done; slices that vary in thickness will cook unevenly. A mandoline slicer set to one-eighth inch produces the most consistent results and the fastest cooking time. A sharp knife and patience work equally well. Layer the sliced potatoes in the prepared casserole dish, spreading them out so they overlap slightly but do not clump into thick piles — even distribution helps the sauce coat every slice and prevents undercooked spots.
Step 3: Make and Pour the Sauce
In a medium bowl, whisk together the condensed cream of chicken soup and the milk until completely smooth. Season with salt, pepper, and any optional seasonings. The mixture should be thick but pourable — like a creamy gravy rather than a thin broth. Pour this sauce slowly and evenly over the layered potatoes, using the back of a spoon to coax it down between the layers and ensure the sauce reaches the bottom of the dish. Every slice should have contact with the sauce.
Step 4: Bake Covered
Cover the casserole dish tightly with aluminum foil. Bake covered at 375°F for 45 to 55 minutes, until the potatoes are very tender when pierced with a fork and the sauce is visibly bubbling around the edges of the dish. Thicker potato slices may need a few extra minutes; very thin slices may be done closer to the 45-minute mark. Begin checking at 45 minutes by inserting a thin knife or fork into the center of the dish — it should slide through without any resistance.
