Preheat the oven to 375°F and coat a 2-quart glass casserole dish with nonstick spray.
Scrub the potatoes, pat them dry, and slice into ⅛–¼ inch rounds.
Arrange the potato slices evenly in the casserole dish, slightly overlapping in a single even layer.
Whisk the condensed cream of chicken soup and milk in a medium bowl for 30 seconds until fully smooth.
Pour the mixture evenly over the potatoes and spread it gently to coat all slices.
Cover the dish tightly with foil and place it on the center oven rack.
Bake for 50 minutes until the potatoes are fork-tender and the sauce is bubbling.
Remove the foil and bake for 15 minutes until the top is lightly browned and the sauce thickens.
Rest the casserole for 10 minutes before serving.
Serve in 6 equal portions, about ¾ cup each.
