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.

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.

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.

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.

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.

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.

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.

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.

Whisk in the flour and cook for 1 to 2 minutes. The mixture should look smooth and foamy but should not brown significantly.

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.

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.

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.

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.

Repeat the noodle, ricotta, vegetable, mozzarella, Parmesan, and sauce layers twice more, using 3 noodles for each layer.

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.

Cover the baking dish loosely with aluminum foil, tenting it slightly so the cheese does not stick. Bake for 35 minutes.

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.

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.

Sprinkle with the optional fresh parsley: Cut into 12 portions and serve warm.