2-3 onions, sliced
3 tbsp unsalted butter
6 russet potatoes, peeled and sliced thinly lengthways
1 2oz can anchovy fillets, drained, chopped – reserve oil
1 1/2 cup heavy cream
Heat up a large skillet on medium heat, add in 2 tbsp butter, then add in the onions. Cook until golden in color.
Preheat oven to 400F. In a buttered 13×9 baking dish, layer 1/3 of the potatoes, 1/2 the onions, 1/2 the anchovies, and salt in pepper in a layer. Add in another 1/3 of the potatoes on top of that, with the rest of the onions and anchovies in layers, with salt and pepper to taste on top of that. Cover with the rest of the potatoes.
Drizzle the potatoes with the reserved oil, about 2 tbsp and dot with the rest of the butter cut into small cubes. Bake the casserole on the middle rack for 10 minutes, then pour 3/4 cup of cream over the potatoes and bake for 20 minutes more. Pour the remaining cream over the rest of it and reduce heat to 300F. Bake for 30 minutes or more until potatoes are cooked. Serve.