Preheat the oven to 375 degrees.
Prep the potatoes and onions by slicing them thinly.
Scatter half of garlic on the bottom of a lightly oiled pie dish.
Put half of the sliced potatoes on top of the garlic, overlapping the slices.
Add half of the onion slices.
Add salt and pepper (go easy on the pepper).
Sprinkle ⅓ cup of cheese.
Repeat the layer of garlic, potatoes, onions, salt, and pepper.
Add the milk mixture evenly over the top.
Add the remaining ⅔ cup of cheese of choice, plus, the ¼ cup of Parmesan cheese.
Cover the pie with foil and bake for 30 minutes.
Uncover and bake for 30 minutes more.
You may want to do a low broil to brown the top.
Cut into wedges to serve.