Step 1: If using fresh spinach, thoroughly rinse under cold water. In a very large pot or Dutch oven, wilt the spinach over medium heat in batches if necessary. If using frozen, ensure it is completely thawed.
Step 2: Transfer the wilted spinach to a colander and press out as much liquid as possible. This step is crucial; drier spinach leads to a creamier dish. Roughly chop the squeezed spinach.
Step 3: Preheat your oven to 375°F (190°C).
Step 4: In a large saucepan or Dutch oven, melt 4 tablespoons of unsalted butter over medium heat.
Step 5: Add the minced garlic and cook for about 1 minute until fragrant, being careful not to burn it.
Step 6: Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux. This will help thicken your sauce.
Step 7: Gradually whisk in the heavy cream, stirring until smooth and thickened, about 3-5 minutes. Make sure there are no lumps.
Step 8: Stir in the chicken broth and bring the mixture to a gentle simmer, allowing it to thicken further. Season with a pinch of nutmeg, salt, and freshly ground black pepper.
Step 9: Remove the sauce from the heat. Stir in the Parmesan cheese, Pecorino Romano cheese, and about half of the shredded Fontina cheese until melted and smooth.
Step 10: Fold in the well-drained and chopped spinach until evenly combined with the cheesy sauce. Taste and adjust seasonings as needed.
Step 11: Transfer the spinach mixture to an 8×8 inch or similar oven-safe baking dish.
Step 12: Sprinkle the remaining Fontina cheese evenly over the top of the spinach mixture.
Step 13: Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and bubbly.
Step 14: Let the dish rest for a few minutes before serving. Serve hot and enjoy your homemade Ruth’s Chris Spinach Rockefeller Copycat!