Preheat the oven: Set your oven to 350°F (175°C). While the oven is heating, take a baking dish and use 10g of semi-salted butter to grease the bottom and sides to prevent sticking.
Prepare the ingredients: Peel and slice 8 medium potatoes into thin rounds—about 1/8 inch thick works best. Also, chop 4-5 scallions, separating the white and green parts for layering.
Make the custard mixture: In a large mixing bowl, whisk together 3 eggs, 100ml of milk, and 100ml of liquid cream until well combined. Next, add in the grated 150g Parmesan cheese and 1 tablespoon of herbs de Provence. Stir until everything is evenly mixed.
Layer the ingredients: Begin by placing a layer of sliced potatoes in the greased baking dish, followed by a sprinkle of chopped scallion whites. Pour a portion of the egg and cheese mixture over the layers to coat. Repeat the process until all potatoes and scallions are used, ensuring the final layer is the cheese mixture for a perfect crust on top.
Bake the dish: Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 45 minutes. After this time, remove the foil and return the dish to the oven to bake for an additional 20-25 minutes or until the top is golden brown and bubbly.
Cool and serve: Once baked, allow the dish to cool for a few minutes before serving. This resting time helps the layers set and makes for easier serving.