Put this dish into the oven before you begin cooking anything else. It will happily bake away while you prepare the rest of the meal.
6 medium potatoes, peeled and very thinly sliced
1 small onion, thinly sliced
1 cup grated Swiss
2 tbsp grated Parmesan
1 cup milk, (or evaporated milk)
In a bowl, toss together the sliced potatoes and onions. The easiest way to do this is by hand. Your child’s hand, to be precise. Arrange half of the potatoes in a well-greased 8 in. (20 cm) square baking dish. Season with salt and pepper, then sprinkle with half the grated Swiss cheese and half the grated Parmesan cheese. Repeat with the rest of the potatoes and onions, and the rest of the cheeses. Pour the milk evenly over the top of everything.
Bake at 375°F (190°C) for 1 hour (or possibly a bit longer), until the potatoes feel soft when you poke a knife into the centre of the dish.