Preparation

Step 1: Prepare the Ricotta Filling

For fresh spinach, heat a large skillet over high heat. Add 2 tablespoons of olive oil and half of the spinach. Season with a pinch of salt and sauté until the spinach wilts and the stems become tender, about 3 minutes. Transfer to a baking sheet and let it cool. Repeat the process with the remaining spinach. If using thawed frozen spinach, skip to Step 2.

Step 2: Combine Cheese and Spinach

While the spinach is cooling, mix the ricotta, mozzarella, Parmesan, and basil in a large bowl. Season with a generous pinch of salt.

Step 3: Incorporate Spinach

Once the spinach has cooled or thawed, squeeze out any excess water. Finely chop the spinach and add it to the cheese mixture. Stir thoroughly and adjust the seasoning for salt. (The ricotta filling will yield about 6 cups.) Cover and set aside until ready to assemble the lasagna. This filling can be made one day ahead and refrigerated. Allow it to come to room temperature before using.

Step 4: Prepare the Béchamel

In a large, heavy-bottomed pot or Dutch oven, melt the butter over medium heat. Once melted, whisk in the flour, reduce the heat to low, and cook for about 15 minutes, stirring regularly to prevent browning. The mixture will change in texture, appearing foamy and sandy as the butter separates. Gradually pour in the milk while whisking vigorously. Increase the heat to medium and continue whisking until the mixture thickens, about 2 minutes. Season with salt, pepper, and nutmeg (if desired).

Step 5: Cook the Béchamel

Reduce the heat to low and cook, whisking regularly, for 10 to 15 minutes, until the sauce is thick, smooth, and has no raw flour taste. If the sauce is lumpy, strain it through a fine sieve or purée it with an immersion blender. You will have about 3½ cups of béchamel. To prevent a skin from forming, cover the surface with parchment or plastic wrap. Set aside until ready to assemble the lasagna. The béchamel can be made one day in advance and refrigerated. Let it return to room temperature before using.

Step 6: Prepare the Lasagna

Position the top rack of the oven about 6 inches from the heating element and preheat to 400°F.

Step 7: Warm the Sauces and Filling

Ensure that the tomato sauce, béchamel, and ricotta filling are at room temperature or warmed before assembling.

Step 8: Prepare the Pasta

Bring a large pot of water to a boil over high heat. Place a large colander in a baking dish next to the stove.

Step 9: Cook the Lasagna Noodles

Season the boiling water generously with salt, aiming for a sea-water-like salinity. Carefully drop the lasagna sheets into the boiling water, adding a few at a time to prevent sticking. Cook for approximately 2 minutes, or until the pasta is light in color and floppy. If the pasta rises above the water, gently press it down using a slotted spoon. Use a sieve to remove the cooked pasta and rinse it under cold water until it is cool enough to handle. Continue cooking, draining, and rinsing the pasta in batches until all sheets are prepared.

Step 10: Coat the Pan

Spoon ¾ cup of béchamel into a 9-by-13-inch baking dish and use a rubber spatula or your hands to evenly coat the bottom and sides with the sauce.

Step 11: Layer the Pasta

Gently squeeze any remaining water from the pasta and arrange two or three sheets on the bottom of the pan. Ensure the pasta covers the bottom with minimal overlap, leaving some edges hanging over the sides to assist in sealing the top later.

Step 12: Add the Ricotta Mixture

Crumble one-third of the ricotta filling over the pasta to create a uniform layer. Top with another layer of pasta, trimming with kitchen shears as needed to avoid overlaps.

Step 13: Add Tomato Sauce

Spread one-third of the tomato sauce over the pasta and cover with another layer of pasta.

Step 14: Add Béchamel and Parmesan

Spread 1 cup of béchamel over the pasta and sprinkle with ¼ cup grated Parmesan. Top with another layer of pasta.

Step 15: Repeat Layers

Continue layering with the remaining ricotta, tomato sauce, béchamel, and Parmesan in the same order: ricotta, tomato sauce, béchamel, Parmesan, followed by a layer of pasta. If using thicker noodles, you may need to adjust the number of layers to fit the pan.

Step 16: Seal and Top

For the final layer, fold the overhanging pasta over to form a sealed top layer. Spread the remaining béchamel over the top and cover the lasagna with parchment paper and tightly wrap it in foil. Place the dish on a baking sheet to catch any spills.

Step 17: Bake the Lasagna

Bake the lasagna for 40 minutes, then carefully remove the foil and parchment. Sprinkle the top with grated mozzarella and return it to the oven. Bake for an additional 20 minutes, or until the surface is golden brown and bubbling.

Step 18: Cool and Serve

Allow the lasagna to cool for at least 15 minutes before slicing and serving. Leftovers can be refrigerated for up to 4 days. For freezing, bake for 30 minutes without browning, then cool and freeze for up to 4 weeks. To reheat, thaw and bake uncovered at 400°F for 25 to 30 minutes, or until golden and bubbling.