1. In a large pan, add the spinach (with a pinch of salt and pepper) and cook over medium/low heat until wilted. Remove from the pan and set aside.
2. In the same pan, heat 1/2 tbsp butter and 1/2 tbsp olive oil. Sauté the shallots along with a pinch of salt, pepper, and red pepper flakes for 3-4 minutes, or until softened. Add the garlic and cook for another 30 seconds.
3. Add the remaining 4 tbsp butter to the pan and melt. Once melted, sprinkle in the flour and whisk constantly for 2-3 minutes to form a roux.
4. Reduce heat to low, gradually whisk in the milk and heavy cream, allowing it to come to a simmer for 2-3 minutes until the sauce thickens. Season with salt, pepper, nutmeg, and cayenne (if using). Taste and adjust the seasonings as needed.
5. Off heat, gradually stir in the Gruyère cheese and 2 oz of Parmesan cheese. Mix until the cheese is fully melted and the sauce is smooth.
6. Stir in the cooked spinach and sour cream (optional) Taste and adjust the seasoning one final time, if needed.
7. Pour the creamed spinach into a baking dish and top with the mozzarella and remaining Parmesan cheese.
8. Place the dish under the broiler for 3-7 minutes (varies by oven), or until the cheese is golden and bubbly.
9. Remove from the oven, let cool slightly, and enjoy!
