Boil potatoes, until just tender but not cooked all the way through, about 20-25 minutes. Drain, cool, peel, and cut into 1/4 inch thick slices.
Preheat oven to 350.
In a greased 9 x 13 dish, layer half of potato slices, then top with half the bacon and onions.
Whisk milk into sour cream and flour until smooth. Season with salt and pepper. Spread half the mixture over the potato layer and then sprinkle with half the cheese. Season with salt and pepper. Repeat layers.
Sprinkle Panko over top and drizzle butter over all. Bake uncovered for 30 minutes until golden and bubbly. Serve hot.