Preheat oven to 170°C.
Heat oil and butter in a large saute pan over medium-high heat. Add the onions, season with salt and pepper and cook until caramelised, about 15 to 20 minutes. Remove from the heat and stir in the sage. Let cool slightly.
Make a layer of potato slices in a 10 by 10 by 2-inch casserole, season to taste with salt and pepper, spread 1/12 of the onion mixture over the potatoes and coat with 2 tablespoons of the cream. Repeat each step to yield 12 layers. Press down gently on the layers, place the dish on a sheet pan and bake, covered for 25 minutes. Remove cover and bake for another 20 to 25 minutes, or until the potatoes are tender and golden brown on top. Let rest 10 minutes before serving.