Preheat the oven to 500 degrees F.
Peel the potatoes and cut them into slices, neither especially thin nor especially thick (approximately 1/2-inch) and put them into a large saucepan with the milk, cream, onion, minced garlic and salt. Bring to the boil and cook at a robust simmer or gentle boil (however you like to think of it) until verging on tender, but not dissolving into mush. The pan might be hell to clean afterward, but any excuse for long, lazy soaking rather than brisk pre-or postprandial scrubbing always appeals to me. And, for what it's worth, I find that when pans are really, dauntingly, stuck with cooked-on gunge, it's more effective to soak them in hot water and detergent (i.e., the stuff you put in the washing machine, though I haven't tried, and don't think I would, with tablets) rather than dish liquid.
Use some of the butter to grease a large roasting pan (15 by 12-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 or until the potato is bubbly and browned on top. Remove, let stand for 10 to 20 minutes and then serve.
This is not the most labor saving way of cooking potatoes, to be sure, but one of the most seductive. And it reheats well as an accompaniment to cold roast pork, or indeed anything in the days that follow.