Equipment
* optional
Ingredients
Produce
- 1200 g sweet potatoes, peeled
- 15 g fresh ginger, peeled and finely grated
- 1 orange, zested
- 2 garlic, minced
Dairy
- 480 ml heavy cream, cold
- 100 g gruyere cheese, freshly grated
- 15 g unsalted butter, softened
Spices & Seasonings
- 8 g kosher salt
- 2 g black pepper, freshly cracked
- 1 g ground nutmeg
Nutrition (per serving)
Method
Preheat the oven to 190°C/375°F. Generously grease the inside of a 9x13-inch baking dish with the softened butter.
In a medium saucepan, whisk together the heavy cream, grated ginger, orange zest, minced garlic, kosher salt, black pepper, and nutmeg. Place over medium heat and bring to a gentle simmer. Once simmering, immediately remove from heat and let steep while you prepare the potatoes.
Using a mandoline slicer or a very sharp chef's knife, carefully slice the peeled sweet potatoes into uniform rounds, about 3 millimeters (1/8 inch) thick.
Arrange the sweet potato slices in the prepared baking dish. Shingle them tightly in slightly overlapping rows, creating multiple layers until all the potatoes are used.
Pour the warm, infused cream mixture evenly over the layered sweet potatoes. Sprinkle the grated Gruyere cheese evenly across the top.
Cover the baking dish tightly with aluminum foil. Bake in the preheated oven at 190°C/375°F for 40 minutes.
Carefully remove the aluminum foil. Continue to bake uncovered for an additional 20 minutes, or until the cream is bubbling, the cheese is deeply golden brown, and the sweet potatoes offer no resistance when pierced with a paring knife.
Remove the gratin from the oven and allow it to rest at room temperature for at least 15 minutes before serving. This rest period allows the sweet potato starches to reabsorb the bubbling cream, setting the sauce.
Chef's Notes
- Do not rinse the sweet potatoes after slicing. The natural surface starches released during slicing are crucial for thickening the cream sauce as the gratin bakes.
- For the cleanest slices, always use a mandoline slicer. However, mandolines are extremely sharp, so a cut-resistant safety glove or the food pusher is mandatory.
- If you prefer a thicker sauce right out of the oven, you can toss the raw sweet potato slices in a tablespoon of all-purpose flour or cornstarch before layering them into the dish.
- The resting phase is non-negotiable. Slicing into a gratin straight from the oven will result in a soupy plate. The sauce needs 15 minutes to set up and stabilize.
Storage
Refrigerator: 4 days — Store tightly covered. The cream sauce will thicken significantly as it chills.
Freezer: 1 month — Wrap tightly in plastic wrap and then foil. Texture of the potatoes may soften slightly upon thawing.
Reheating: Cover with foil and bake at 175 C / 350 F for 20 to 25 minutes until heated through and bubbling at the edges.
