French Casserole Potatoes

Ingredients:
3 T unsalted butter
1 T 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

Preparing the potatoes:
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 for a crisp top.

Serves 6