This cozy winter supper combines thinly sliced potatoes layered into a glass casserole dish and drenched in a rich pour-over of heavy cream, sweet caramelized onions, nutty Parmesan, and fresh thyme. Baking up tender like scalloped potatoes with the golden, bubbly crust of a classic gratin, it is the ultimate comfort side for roasts, chops, or weeknight dinners.
Equipment & Ingredients
Equipment Required
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9×13-inch glass casserole dish
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Large skillet (for caramelizing onions)
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Mandoline slicer or sharp chef’s knife
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Aluminum foil
Component Breakdown
| Section | Ingredient | Quantity | Functional & Culinary Role |
| Potato Base | Russet or Yukon Gold potatoes | 3 lbs (1.4 kg) | High-starch/creamy spuds sliced ⅛-inch thick for even layering. |
| Butter or cooking spray | 1 tbsp | Greases the glass dish to prevent sticking. | |
| Caramelized Onion Layer | Yellow onions | 2 large, thinly sliced | Slow-cooked alliums providing deep, natural sweetness. |
| Unsalted butter | 2 tbsp | Sautéing fat that carries aromatic compounds. | |
| Kosher salt | ½ tsp (divided) | Draws out moisture from onions during sweating. | |
| Granulated sugar (optional) | 1 tsp | Accelerates surface caramelization. | |
| Olive oil (optional) | 1 tbsp | Prevents scorching if onions dry out. | |
| Creamy Pour-Over Matrix | Heavy whipping cream | 2 cups (480ml) | Rich lipid liquid base that thickens as potato starches cook. |
| Grated Parmesan cheese | 1 cup + 2 tbsp (divided) | Sharp, savory umami binder and crispy golden gratin crust. | |
| Fresh thyme | 2 tsp chopped (or 1 tsp dried) | Earthy, floral herb accent. | |
| Kosher salt & black pepper | 1 tsp salt, 1 tsp black pepper | Primary flavor and seasoning regulators. |
🧪 Food Science: Starch Gelatinization & Lactose Caramelization
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Thermal Starch Hydrolysis: Slicing potatoes to a uniform ⅛-inch thickness exposes surface starches (amylose and amylopectin). As heavy cream simmers around the potato slices at $180^\circ\text{F}\text{–}200^\circ\text{F}$, these starches hydrate and swell, thickening the cream naturally into a velvety sauce without requiring a flour roux.
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Maillard Browning via Slow Caramelization: Cooking sliced onions slowly over 20 to 25 minutes breaks down complex fructan sugars into simple fructose and glucose. These react with natural amino acids to create rich, caramelized flavor compounds that infuse the entire dairy matrix.
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Steam Trapping vs. Uncovered Gratin Crisp: Baking covered with foil for the first 45 to 55 minutes traps escaping water vapor, creating a high-humidity environment that cooks the dense potato discs to fork tenderness. Uncovering for the final 15 to 20 minutes allows surface moisture to evaporate so milk solids and Parmesan proteins can undergo rapid Maillard browning.
Step-by-Step Instructions:
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