Step 1: Prepare the zucchini. Lay zucchini slices in a single layer on paper towels or a clean kitchen towel. Sprinkle generously with 1 teaspoon of salt and let sit for 20-30 minutes to draw out excess moisture. Blot thoroughly dry with more paper towels.
Step 2: Sauté aromatics. In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Remove from heat and set aside.
Step 3: Prepare the ricotta mixture. In a medium bowl, combine the ricotta cheese, egg, 1/2 cup Parmesan cheese, chopped parsley, and 1/4 teaspoon black pepper. Mix well until everything is evenly combined.
Step 4: Make the pesto béchamel sauce. In a large saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 4 tablespoons of flour and cook for 1-2 minutes, stirring constantly, to create a roux. Gradually whisk in the warm milk, a little at a time, ensuring no lumps form. Continue whisking until the sauce thickens to a creamy consistency, about 5-7 minutes. Remove from heat and stir in the pesto, nutmeg, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until fully combined.
Step 5: Preheat oven and assemble. Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish. Spread a thin layer of pesto béchamel sauce on the bottom of the dish.
Step 6: Layer the lasagna. Arrange a single layer of dried zucchini slices over the béchamel. Spread half of the ricotta mixture over the zucchini, then sprinkle with 1 cup of mozzarella cheese. Drizzle with more pesto béchamel sauce. Repeat with another layer of zucchini, the remaining ricotta mixture, 1 cup of mozzarella, and more béchamel.
Step 7: Final layers. Top with a final layer of zucchini, the remaining pesto béchamel sauce, and then sprinkle generously with the remaining 2 cups of mozzarella cheese and extra grated Parmesan. Gently press down on the lasagna to ensure all layers are compact.
Step 8: Bake. Cover the baking dish loosely with aluminum foil (you may want to tent it slightly to prevent cheese from sticking). Bake for 25 minutes. Remove the foil and bake for another 15-20 minutes, or until the top is golden brown and bubbly. Let the Pesto Béchamel Zucchini Lasagna rest for 10-15 minutes before slicing and serving. This allows the layers to set and prevents it from falling apart.