Cook the nine lasagna noodles according to package instructions until al dente. Drain and lay them flat on a lightly oiled baking sheet to prevent sticking.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Toss in the fresh spinach and cook until just wilted, about 2-3 minutes. Season lightly with salt and pepper. Remove from heat and set aside.
In a medium saucepan over medium heat, melt 4 tablespoons of butter. Whisk in ¼ cup all-purpose flour and cook for 1-2 minutes, stirring constantly until it turns a light golden color.
Gradually whisk in 3 cups of warm milk, continuing to whisk until the mixture thickens and becomes creamy. Season with salt, pepper, and Italian seasoning. Remove from heat once it reaches a smooth, pourable consistency.
In a large bowl, combine the ricotta cheese, shredded chicken, half of the mozzarella, and half of the Parmesan. Season with a pinch of salt, pepper, and a little Italian seasoning. Gently fold in the sautéed spinach and garlic mixture.
Preheat your oven to 375°F (190°C). Spread a thin layer of béchamel sauce on the bottom of your baking dish. Lay three lasagna noodles over the sauce, slightly overlapping.
Spread one-third of the chicken and cheese mixture over the noodles, then drizzle with béchamel sauce. Repeat this layering process two more times: noodles, filling, sauce.
Finish with a final layer of noodles topped generously with the remaining béchamel sauce, mozzarella, and Parmesan cheeses.
Cover the lasagna tightly with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 15 minutes until the top is golden and bubbly.
Let the lasagna rest for at least 10 minutes before slicing – this helps everything set beautifully and makes cutting easier.