Prepare the oven and baking dish: Preheat the oven to 375°F (190°C). Lightly coat a 9 x 13-inch (23 x 33 cm) baking dish with nonstick cooking spray. This ensures that the lasagna does not stick to the dish, making it easier to serve once baked.

Cook the lasagna noodles: Bring a large pot of water to a rolling boil and add 1 tablespoon fine salt. Cook the lasagna noodles according to the package directions until just al dente. Drain and lay them flat on a clean kitchen towel or lightly oiled baking sheet so they do not stick together. This step is crucial to prevent the noodles from clumping, which can make assembly difficult.

Brown the mushrooms: Heat 1 tablespoon (15 ml) olive oil in a 12-inch skillet over medium-high heat. Add half of the sliced mushrooms and cook for 5 to 7 minutes, stirring occasionally, until their liquid evaporates and the edges begin to brown. Transfer to a bowl and repeat with the remaining tablespoon of olive oil and mushrooms. Browning the mushrooms enhances their flavor and prevents excess moisture in the filling.

Cook the onion and garlic: Return all of the mushrooms to the skillet and add the diced onion. Cook over medium heat for 4 to 5 minutes, until the onion becomes translucent. Add the 4 minced garlic cloves and cook for 30 seconds. This step builds a flavorful base for the filling.

Wilt the spinach: Add the spinach in batches, stirring as each addition wilts. Continue cooking until the spinach is fully wilted and no liquid remains in the bottom of the skillet, about 3 to 4 minutes. Properly wilting the spinach ensures that the filling remains rich and does not become watery.

Season the vegetable filling: Stir in the Italian seasoning, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Remove the skillet from the heat. The mixture should look moist but not watery. Set aside. This seasoning step enhances the overall flavor of the filling.

Make the ricotta filling: In a large bowl, combine the ricotta, egg, 1/2 cup (45 g) Parmesan, parsley, 1/4 teaspoon salt, and 1/4 teaspoon black pepper. Stir until smooth and evenly combined. This filling adds creaminess and richness to the lasagna.

Start the white sauce: Melt the butter in a 3- to 4-quart saucepan over medium heat. Add the 2 garlic cloves and cook for about 30 seconds, until fragrant. This step infuses the butter with garlic flavor, which will enhance the overall taste of the lasagna.

Make the roux: Whisk in the flour and cook for 1 to 2 minutes. The mixture should look smooth and foamy but should not brown significantly. This roux is the base for the creamy white sauce.

Add the milk: Slowly pour in the warmed milk while whisking continuously. Begin with about 1 cup (240 ml) and whisk until smooth before gradually adding the remainder. Adding the milk gradually helps prevent lumps from forming in the sauce.

Thicken the sauce: Add the 1/2 teaspoon salt, 1/4 teaspoon black pepper, and nutmeg. Cook over medium heat for 5 to 7 minutes, whisking frequently, until the sauce gently simmers and thickens enough to coat the back of a spoon. This thickened sauce will contribute to the creamy texture of the lasagna.

Begin assembling the lasagna: Spread about 1 cup (240 ml) of the white sauce over the bottom of the prepared baking dish. Arrange 3 lasagna noodles over the sauce. This initial layer helps to prevent the noodles from sticking to the bottom of the dish.

Build the first layer: Spread one-third of the ricotta mixture over the noodles. Top with one-third of the spinach-mushroom mixture, 3/4 cup (85 g) mozzarella, 2 tablespoons (about 11 g) Parmesan, and about 3/4 cup (180 ml) white sauce. Layering in this way ensures even distribution of flavors and textures.

Repeat the layers: Repeat the noodle, ricotta, vegetable, mozzarella, Parmesan, and sauce layers twice more, using 3 noodles for each layer. This repetition builds the structure of the lasagna and ensures that every bite is flavorful.

Finish the top: Arrange the final 3 noodles over the filling. Spread the remaining white sauce evenly over the noodles. Top with the remaining 3/4 cup (85 g) mozzarella and remaining Parmesan. This final layer of cheese will create a deliciously golden and bubbly top.

Bake covered: Cover the baking dish loosely with aluminum foil, tenting it slightly so the cheese does not stick. Bake for 35 minutes. Covering the dish helps to keep moisture in, ensuring the lasagna cooks evenly.

Brown the cheese: Remove the foil and continue baking for 15 to 20 minutes, until the sauce bubbles around the edges and the cheese is melted with light golden spots. This step gives the lasagna a beautiful, appetizing appearance.

Rest before slicing: Remove the lasagna from the oven and let it stand uncovered for 20 minutes. This resting time allows the sauce and cheese to firm enough for cleaner portions. Cutting too soon can cause the layers to slide apart.

Garnish and serve: Sprinkle with the optional fresh parsley. Cut into 12 portions and serve warm. The garnish adds a touch of color and freshness to the dish.