Begin by peeling and washing the potatoes.

Using your mandolin, slice all the potatoes to roughly 1/4" of thickness

Slice the onions to a similar thickness.

In a baking dish, place 2 tablespoons each of the butter and oil.

Start ringing the potatoes and onions around the outside of the bowl.

After every three potatoes, add a sliced onions layer.

Work all the way around until a circle is created around the outside of the dish.

Move to the center and try to create a ring again following the same potatoes and onion distribution.

Taking a brush, brush the remaining butter and oil on the top of the potatoes and onions.

Sprinkle the salt and pepper evenly on the top of the potatoes.

Bake in an oven at 450 degrees for approximately 50 minutes.

Serve and enjoy!