Preheat oven to 375°F.

Rub a 13x9 baking dish with butter.

Layer half of the potato slices in dish, overlapping slightly.

Spread the spinach on top of the potato slices.

Add the crumbled feta cheese over the spinach.

Layer the rest of the potato slices on top.

In a separate bowl, whisk together heavy cream, eggs, salt, and pepper.

Pour the cream mixture into dish to cover potatoes; cover with foil.

Bake for 40 minutes, until potatoes are tender.

Remove foil and bake until top is golden, about 35-40 minutes.

Let rest for 15 minutes before serving.