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

Directions

•Preheat the oven to 300 degrees F.

•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.