Preheat oven to 350F (180C) and grease a 11 x 7-inch (2L) casserole dish.
In a medium, heavy-bottomed pot, cook bacon until crisp and remove to drain. Over medium-high heat, sauté onion in bacon drippings until tender and lightly browned, about 5 minutes. Add garlic and cook 1 minute more. Add milk and bring up to a simmer. Remove from heat, whisk in sour cream and season lightly.
To assemble, spoon a little of the milk mixture in the bottom of the prepared baking dish and arrange a single layer of sliced potatoes over. Spoon a layer of milk mixture over potatoes and sprinkle with a bit of reserved bacon, green onion and cheddar cheese. Top with another layer of potatoes and continue layering with milk mixture, bacon, green onions, Cheddar and potatoes, finishing with a final sprinkling of cheddar. Place a piece of parchment over top layer and cover dish with foil (or a lid). Bake, covered, for 45 minutes, then remove foil (or lid) and parchment and cook another 20 to 30 minutes, until a knife inserted in the center of the dish yields easily. Let cool for 15 minutes before serving.