Start the sauce by preheating broiler to high heat. Put the peppers and onion on a baking sheet and broil until the peppers are black and the onion is charred. Remove, and transfer the peppers to a container and cover. Set aside while you make the potatoes.
Preheat oven to 425 degrees F.
Using a mandolin, slice the potatoes and onion as thin as you can.
Brush the inside of square baking dish with oil, butter, or cooking spray.
Layer the potatoes, onion, and garlic in the dish, seasoning with salt and pepper between each layer.
Whisk the milk with the flour and pour over the potatoes.
Drizzle with melted butter and cover tightly with foil.
Bake for 45 minutes. Remove the foil and continue baking for 10 more minutes.
While the potatoes are baking, remove the peppers from the sealed container. Using paper towels or a clean kitchen towel, rub the skin off the peppers.
Add to a blender with the onion, garlic, tomatillos, cilantro and broth.
Blend until smooth, and season with salt and pepper.
Transfer to a saucepan and bring to a simmer.
Cut the potatoes into 16 squares. Transfer to a plate and pour the sauce over top. Sprinkle with cheese and sour cream if desired.
Notes
Storing leftovers: Leftovers can be stored separately in an airtight container in the fridge.