Follow these detailed steps to assemble a perfect Turkey and Spinach Lasagna. The process is broken down into making the sauces, assembling the layers, and baking to golden perfection. Taking your time with each stage is the key to an unforgettable result.

Step 1: Prepare the Turkey and Spinach Sauce

1. Sauté Aromatics: Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the finely chopped yellow onion and cook, stirring occasionally, for 5-7 minutes, or until it becomes soft and translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.
2. Brown the Turkey: Add the ground turkey to the pot. Break it up with a wooden spoon and cook until it is no longer pink. Drain any excess fat if necessary.
3. Season the Sauce: Season the cooked turkey with 1 teaspoon of salt, ½ teaspoon of black pepper, dried oregano, dried basil, and optional red pepper flakes. Stir well to ensure the meat is evenly coated with the spices.
4. Simmer: Pour in the can of crushed tomatoes. Stir everything together, scraping up any browned bits from the bottom of the pot. Bring the sauce to a gentle simmer, then reduce the heat to low, cover, and let it cook for at least 20 minutes to allow the flavors to meld together. A longer simmer (up to an hour) will result in an even richer sauce.
5. Wilt the Spinach: During the last 5 minutes of simmering, add the fresh spinach to the pot in batches, stirring it into the sauce until it wilts down completely. Once all the spinach is incorporated, turn off the heat and set the sauce aside.

Step 2: Create the Béchamel Sauce

1. Warm the Milk: Gently warm the milk in a separate saucepan or in the microwave. It should be warm to the touch, not boiling. This step is crucial for a smooth sauce.
2. Make the Roux: In a medium saucepan, melt the unsalted butter over medium heat. Once melted, whisk in the all-purpose flour. Cook this mixture (the roux), whisking constantly, for about 2 minutes. This cooks out the raw flour taste and is the foundation for your sauce.
3. Incorporate the Milk: Slowly begin to pour the warm milk into the roux, about a half-cup at a time, while whisking vigorously and constantly. Do not add the next batch of milk until the previous one is fully incorporated and the sauce is smooth. Continue this process until all the milk has been added.
4. Thicken and Season: Continue to cook the sauce, stirring frequently, for about 5-8 minutes, or until it has thickened enough to coat the back of a spoon. Remove from the heat and stir in the salt, white pepper, and a pinch of nutmeg. Set aside.

Step 3: Assemble the Lasagna

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). If you are using lasagna noodles that require boiling, cook them now according to package directions until al dente. Drain and lay them flat on a baking sheet lined with parchment paper to prevent sticking. Lightly grease a 9×13 inch baking dish.
2. First Layer: Spread a thin layer of the turkey and spinach sauce on the bottom of the prepared baking dish. This prevents the first layer of noodles from sticking and burning.
3. Second Layer: Arrange a single layer of lasagna noodles over the sauce.
4. Third Layer: Spread about one-third of the remaining turkey and spinach sauce over the noodles.
5. Fourth Layer: Drizzle about one-quarter of the béchamel sauce over the turkey sauce.
6. Fifth Layer: Sprinkle with a generous layer of grated mozzarella cheese and a light dusting of Parmesan cheese.
7. Repeat: Repeat the layering process (Noodles -> Turkey Sauce -> Béchamel -> Cheeses) two more times, creating three full layers of filling.
8. Final Layer: Top with the final layer of noodles. Spread the remaining béchamel sauce over the top, making sure to cover the noodles all the way to the edges. This protects the noodles from drying out and creates a creamy top. Sprinkle generously with the remaining mozzarella and Parmesan cheese.

Step 4: Bake and Rest

1. Bake Covered: Lightly spray a piece of aluminum foil with cooking spray (to prevent the cheese from sticking) and cover the baking dish. Bake in the preheated oven for 30 minutes.
2. Bake Uncovered: Remove the foil and bake for another 20-25 minutes, or until the sauce is bubbly and the cheese on top is melted, golden brown, and slightly crispy in spots.
3. Rest the Lasagna: This is a critical step! Remove the lasagna from the oven and let it rest on a wire rack for at least 15-20 minutes before slicing and serving. This allows the layers to set, ensuring clean, beautiful slices instead of a soupy mess.
4. Garnish and Serve: Garnish with fresh chopped basil or parsley before slicing and serving.