Sauté olive oil and a dash of balsamic vinegar in a medium-hot pan with a dash of salt, until the onions are caramel in color. Set aside.
Place thinly sliced potatoes in a layer in the bottom of a baking dish.
Whisk the egg in a small bowl and then spread it over the potato crust.
Cook in the oven for 15 minutes or until potatoes are slightly crispy and then remove. On top of the potato crust, spread ricotta, onions, and arugula.