If you are roasting a joint of meat, you can finish cooking it on a trivet over this onion cake so that the juices get absorbed into the potatoes and onions ... delicious.
Preheat the oven to 200°C (fan oven 180°C), gas mark 6. Peel and thinly slice the potatoes, then rinse briefly in water and pat dry on a clean tea towel. Put them a bowl, season with salt and pepper, and mix with the melted butter.