Heat oven to 450°F. Place melted butter in the bottom of a large bowl. Add the thyme, salt, pepper, and potato slices; toss evenly to coat them.
Tip everything into a 9x13-inch metal baking dish. Roast potatoes for 5 minutes. Use a thin metal fish spatula to loosen potatoes and turn them over. Carefully pour stock into the pan and scatter the garlic cloves over the potatoes. Roast for 30 to 35 minutes more, until potatoes are fully tender.
Remove from oven. Transfer to a serving platter and serve at once with any remaining cooking liquid, if desired.