1-Getting started with this spinach gratin recipe: Begin by preheating your oven to 425 degrees Fahrenheit, which helps create that bubbly, golden finish. Follow the directions carefully to blend flavors and textures just right for a dish that’s ideal for holidays or quick dinners.

2-First: melt the 3 tablespoons of whipped butter in a heavy-bottomed pan over medium heat, then sauté the 1 cup of finely chopped onion until it’s translucent, about 10 to 12 minutes. Next, add the 1/4 cup of all-purpose flour or gluten-free flour mix and 1/4 teaspoon of fresh grated nutmeg, cooking for 2 minutes while stirring to build a flavorful base. Pour in the 3 cups of 2% milk and cook for 5 to 7 minutes until the mixture thickens, creating a smooth sauce.

3-After that, squeeze out as much liquid as possible from the 3 pounds of defrosted frozen chopped spinach using a dish towel or paper towels to avoid a watery result. Stir the spinach into the sauce, then mix in half of the 3/4 cup freshly grated Parmesan cheese, along with the 1 tablespoon of kosher salt and 1/2 teaspoon of freshly ground black pepper for seasoning. Transfer the mixture to a large 9×12-inch baking dish, sprinkle the remaining Parmesan and the 1/2 cup of shredded Gruyere or Swiss cheese on top, and bake for 20 minutes until hot and bubbly.