Prepare Oven and Topping : Position a rack in the center of your oven and preheat to 375°F (190°C). In a small bowl, combine the crushed pork rinds with half of the shredded Gruyère and half of the grated Parmesan. Set this crunchy topping aside for later.
Sauté the Aromatics : Warm the olive oil in a 10-inch cast-iron skillet over medium heat. Add the finely chopped leek and cook, stirring occasionally, for 3–4 minutes until it has softened. Stir in the pressed garlic and dried thyme, continuing to cook for another 45 seconds until fragrant.
Heat the Spinach : Add the thoroughly squeezed spinach to the skillet. Stir constantly for 2–3 minutes, allowing the heat to evaporate any excess moisture and warm the spinach through.
Build the Creamy Base : Lower the heat to low. Add the softened mascarpone cheese, heavy cream, sea salt, pepper, and grated nutmeg to the skillet. Stir continuously until the mascarpone fully melts into the cream, creating a smooth, velvety sauce that envelops the spinach.
Incorporate the Cheeses : Remove the skillet from the heat. Fold in the remaining half of the Gruyère and Parmesan cheeses, stirring just until they are melted and integrated into the mixture.
Bake to a Bubbly Finish : Smooth the surface of the spinach mixture with your spatula. Sprinkle the reserved cheese and crumb topping evenly across the surface. Bake for 20–25 minutes, until the edges are bubbling gently and the center is thoroughly heated.
Final Crisp & Rest : For a richer color and crispier top, switch the oven to a high broil setting for the final 1–2 minutes. Keep a close eye on it to prevent scorching. Let the gratin rest in the skillet for 5 minutes before serving. A thermometer inserted into the center should read at least 165°F (74°C).
