Preheat oven to 400ºF. Spray standard muffin cups with cooking spray.: As the oven warms the air will carry a faint metal warm scent and the interior light will glow, signaling it is ready. Preheating ensures the stacks start cooking immediately, promoting crisp edges. Spraying the muffin cups prevents sticking and helps you lift the portions out cleanly after baking. A frequent misstep is skipping preheat, which can lead to longer bake times and less even browning; always wait until the oven reaches the correct temperature before inserting the pan.
Clean and peel the potatoes with a vegetable peeler. Thinly slice potatoes with a mandoline slicer.: When you peel the potatoes the skin comes away in smooth ribbons, and the exposed flesh looks pale and matte. Using a mandoline produces paper thin slices that fan when lifted, which is crucial for even cooking. You should hear a soft whispering sound as slices separate and feel cool, moist surfaces. One trap is pressing too hard on the mandoline, which can produce jagged edges or uneven thickness; use steady, gentle pressure and a guard for safety.
Place 2 slices in each muffin cup and season with garlic salt, ground pepper, and dried rosemary. Continue adding potatoes, seasoning every two slices, until cups are filled.: As you build each stack, the layers compress and create a subtle fragrance of raw potatoes and herbs. Seasoning every couple slices ensures the flavor penetrates each layer instead of sitting only on the surface. You will notice small gaps close as the stack gets higher, and the topmost edge begins to show the sliced ridges. A common error is over seasoning the bottom only, which gives uneven taste; distribute the seasonings evenly for balanced bites.
Pour 1 tablespoon heavy cream over each potato stack.: As the cream hits the top of a stack it will bead and then slowly sink between slices, carrying herbs with it. That initial glossy sheen is a good visual cue that you used the right amount. The cream helps steam the inner layers while also contributing to browning at the surface. Avoid pouring too much at once, as excess liquid can pool and prevent crisp edges from forming.
Bake until potatoes are golden brown and tender when pierced with a knife, 30-35 minutes.: During baking you will hear occasional gentle bubbling and see steam escaping from the pan, and the tops will develop a warm, toasty brown. A knife should slide through with little resistance, indicating tender centers. The aroma will deepen into roasted, herb scented notes. If the edges brown too quickly, move the pan to a lower rack; rushing this step with higher heat risks burnt edges and raw centers.
Run a thin knife around each gratin and gently lift the potato stacks from the muffin pan.: After resting briefly the stacks become firmer and easier to release. Running a thin knife around the edge loosens any caramelized bits, and you should feel a slight give as the stack releases from the cup. Lift gently and the bottom will show a golden lace where the edges crisped. Forcing them out too soon can break the stack, so be patient and loosen fully before lifting.