Preheat oven to 175°C.
In a medium, heavy-bottomed pot, cook bacon until crisp and remove to drain. Brush an 11-x-7-inch casserole dish with some of the bacon drippings.
Over medium-high heat, sauté onion in the remaining 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 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. Dollop small amounts of sour cream on top.
Top with another layer of potatoes and continue layering with milk mixture, bacon, green onions, Cheddar cheese, sour cream and potatoes, finishing with a final sprinkling of Cheddar cheese and a few dollops of sour cream.
Place a piece of parchment paper, cut to the size of the dish, 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.