Preheat oven to 375 degrees.
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 mixing bowl, whisk together the heavy cream, eggs, and salt and pepper.
Pour the cream mixture into dish to cover potatoes; cover with foil.
Bake for 40 minutes, or until potatoes are tender when pierced with a fork.
Remove foil and continue to bake until top of gratin is golden, 35 to 40 minutes.
Let rest 15 minutes before cutting and serving.