Preheat oven to 325°F. Generously butter four 8-ounce ramekins. Bring a large pot of lightly salted water to a boil. Add the spinach and cook until tender, about 3 minutes. Drain and rinse under cold water. A handful at a time, squeeze out the excess moisture from the spinach. Chop coarsely. You should have about 1 cup chopped spinach.

In a medium bowl, whisk together the cream and yolks until well combined. Stir in the cooked spinach, cheese, salt, pepper, and nutmeg. Ladle or pour equal quantities of the custard into the ramekins, and place them in a roasting pan.

Custard cooks best when cooked gently surrounded by hot water, called, in France, a bain marie. Create this by adding enough hot water to the pan to come halfway up the sides of the ramekins. Bake for 35 minutes, or until it sets the custard when given a slight shake, or when a sharp knife inserted comes out clean. Remove ramekins from the pan and let stand for 5 minutes. Run a knife around the inside of each ramekin and invert on separate plates to unmold. Serve hot.