Ingredients

• 3 tablespoons butter
• 1 tablespoon olive oil
• 4 onions, sliced
• 2 pounds potatoes, thinly sliced
• Kosher salt and freshly ground black pepper
• Fresh thyme sprigs, leaves picked
• 2 cups beef stock

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Instructions

1. Preheat the oven to 300 degrees F.
2. Melt half the butter with the olive oil in a saute pan, and gently fry the onions until soft and lightly golden, about 15 minutes. Spread half the onions in the bottom of a casserole. Lay a layer of sliced potatoes on top, season with salt and pepper, and scatter with thyme leaves. Build another layer of onions, then a final one of potatoes, and finally pour over the stock. Cover the pan with foil, and bake until all the liquid has been absorbed, 2 to 3 hours, removing the foil for the last hour if you like a crisp top.