Preheat the oven to 240°C.

Peel the potatoes and cut them into slices (approx. 1 cm) and put them into a large saucepan with the milk, cream, onion, minced garlic and salt. Bring to the boil and cook at a gentle boil until verging on tender, but not dissolving into mush.

Use some of the butter to grease a large roasting tin (37 x 30 cm) and pour the milk and potato mixture into it. Dot with remaining butter and cook in the oven for 15 minutes or until the potato is bubbly and browned on top. Remove, let stand for 10-20 minutes and then serve.