Preheat oven to 350°F
Heat oil in a large heavy pan, over medium heat. Add the spinach and sauté 3 to 4 minutes, or until all the leaves have soften. Transfer to a colander and drain.
In the same pan, add butter and melt. Add the onions and sauté for 3 to 4 minutes, or until softened. Add garlic and sauté with the onions for about 15 to 20 seconds, making sure not to burn. Place the spinach back in the pan with the onion mixture and stir.
Add the heavy cream, cream cheese, salt, black pepper, and nutmeg. Add ½ cup of the Fontina and all of the Parmesan cheese; mix well. Top with the remaining ½ cup of Fontina and Transfer to an oven-proof baking dish. Bake for 15 to 20 minutes, or until cheese is golden and bubbly.
Remove from the oven, let cool for 5 to 10 minutes before serving. Enjoy!
