Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish.
Melt the butter in a skillet over medium heat, and saute the onion and pancetta until onion is tender and pancetta is cooked through.
Alternately layer the potato slices, salt and pepper, pancetta and onion mixture, spinach, and cheese in the prepared baking dish. Pour the heavy cream over all.
Cover, and bake 1 hour in the preheated oven. Uncover, and continue baking 30 minutes, until bubbly and lightly browned.
