Creamy Caramelized Onion & Parmesan Ranch Potato Gratin
Ingredients:
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For the Onions:
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2 large sweet onions, thinly sliced into half-moons
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2 tablespoons butter
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Pinch of salt
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For the Potatoes:
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3 lbs Yukon Gold potatoes, peeled and sliced into ⅛-inch rounds (a mandoline helps immensely)
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For the Sauce:
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1 ½ cups heavy cream
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½ cup whole milk
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1 packet (1 oz) dry ranch seasoning mix
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1 ¼ cups freshly grated Parmesan cheese, divided
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½ teaspoon black pepper
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For the Top:
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¼ cup panko breadcrumbs (optional, but gives a great crust)
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Fresh thyme leaves or chives, for serving
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Instructions:
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Caramelize the Onions: Melt the butter in a large skillet over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring every few minutes, for 20–25 minutes, reducing the heat to medium-low as needed, until they turn a deep, jammy golden brown. If the pan gets dry, add a splash of water to deglaze. Set aside.
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Preheat & Prep: Preheat oven to 375°F. Butter a 9×13 baking dish (or a 10-inch cast-iron skillet) generously.
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Make the Ranch Cream: In a large bowl, whisk together the heavy cream, milk, dry ranch seasoning, and black pepper until the ranch is dissolved. Stir in 1 cup of the Parmesan.
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Layer the Gratin: Arrange one overlapping layer of potato slices in the dish. Scatter ⅓ of the caramelized onions over the potatoes. Ladle about ⅓ of the cream mixture over the top, making sure it seeps down into the slices. Repeat two more times with the remaining potatoes, onions, and cream. You should end with a layer of cream on top.
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Top & Bake: Cover the dish tightly with foil and bake for 45 minutes. Remove the foil, then sprinkle the remaining ¼ cup Parmesan and the panko breadcrumbs (if using) evenly over the surface.
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Finish Baking: Bake uncovered for another 25–30 minutes, until the top is deeply golden, the edges are bubbling furiously, and a knife slides through the potatoes without resistance.
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Rest & Serve: Let the gratin stand for at least 15 minutes before serving — this is critical for the sauce to set up into a luxurious cream rather than a runny liquid. Scatter fresh thyme or chives over the top.
Why This Works:
The dry ranch seasoning dissolves directly into the cream, so you get the tangy buttermilk, garlic, and herb notes in every bite without any separated lumps. Meanwhile, the deeply caramelized onions melt into soft, sweet strands that disappear into the layers.
Serving Suggestion: Serve as a main dish with a sharp arugula salad, or as the star side next to a simple roasted chicken or seared steak. It’s rich enough to be the centerpiece of a winter supper.