Preheat the oven to 375°F (190°C). Bring a large pot of salted water to a boil and cook the lasagna noodles until al dente according to the package directions. Drain well. Toss with a little olive oil if necessary to prevent sticking.

Shred the cooked chicken into bite-sized pieces using your hands or two forks. Set aside.

To prepare the spinach cream sauce, melt the butter in a saucepan over medium heat. Add the diced onion, minced garlic, garlic powder, and salt. Cook for 2–3 minutes, until the onion becomes translucent and the garlic is fragrant.

Whisk in the flour until it has been completely absorbed by the butter. Gradually pour in the chicken broth and heavy cream while whisking continuously to create a smooth sauce.

Bring the sauce to a gentle boil, then reduce the heat to low. Simmer for 2–3 minutes, whisking frequently, until thick and creamy.

Remove from the heat and stir in the Parmesan until melted. Add the chopped spinach and stir until completely wilted. Set the sauce aside to cool slightly.

In a large bowl, combine the ricotta, egg, 2½ cups mozzarella, salt, and black pepper. Mix until evenly combined.

Spread about ½–¾ cup spinach cream sauce over the bottom of a deep 9×13-inch baking dish. Arrange three cooked lasagna noodles over the sauce.

Spread half of the ricotta mixture over the noodles. Add half of the shredded chicken, followed by about one-third of the remaining cream sauce.

Add another three noodles. Layer with the remaining ricotta mixture, remaining chicken, and another third of the sauce.

Finish your White Chicken Lasagna with the final three noodles and remaining spinach cream sauce. Sprinkle the reserved 1½ cups mozzarella evenly over the top.

Bake for 25–30 minutes, until the sauce is bubbling around the edges and the mozzarella is melted with lightly golden spots.

Remove from the oven and rest for 15–20 minutes before slicing. Garnish with chopped parsley if desired and serve warm.