Peel the potatoes and cut the into 1/2-inch slices. Put them into a large sauce pan with the milk, cream, onion, minced garlic and salt. Bring to the boil and cook at a robust simmer or gentle boil until verging on tender, but not dissolving into mush. Discard the onion
Grease a large roasting pan (15x12 inches) and then pour the almost sludgy milk and potato mixture into it. Dot with remaining butter and cook in the oven for 15 minutes (longer with my oven) or until the potato is bubbly and browned on top. Remove, and let stand for 10-20 minutes and then serve
Notes
I've tried substituting heavy cream with equal amount of coconut milk and it was delicious too