Preheat the oven to 375°F.
Bring a large pot of salted water to a boil. Cook the lasagna noodles according to the package directions until al dente. Drain carefully and return them to the pot. Add a little olive oil if desired to help prevent the noodles from sticking.
Shred the white and dark meat from the rotisserie chicken using your hands or two forks. Set aside.
To make the spinach cream sauce, melt the butter in a 3-quart 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 is completely absorbed into the butter.
Slowly pour in the chicken broth and heavy cream while whisking constantly. Continue whisking until the mixture is smooth and no flour lumps remain.
Bring the sauce to a gentle boil, then reduce the heat to low. Simmer for 2–3 minutes, whisking frequently, until the sauce thickens.
Remove the sauce from the heat. Stir in the Parmesan cheese until melted and fully incorporated. Add the chopped spinach and stir until the spinach has wilted completely. Set the sauce aside to cool slightly.
Prepare the ricotta mixture by combining the ricotta, egg, 2½ cups of the shredded mozzarella, salt, and black pepper in a large bowl. Stir until evenly combined.
To assemble the white chicken lasagna, spread about ½–¾ cup of the spinach cream sauce over the bottom of a deep 9×13-inch baking dish.
Place three cooked lasagna noodles evenly over the sauce.
Spread half of the ricotta mixture over the noodles.
Sprinkle half of the shredded chicken evenly over the ricotta layer.
Spoon one-third of the remaining spinach cream sauce over the chicken.
Add three more lasagna noodles. Spread the remaining ricotta mixture over the noodles, followed by the remaining shredded chicken.
Spoon another one-third of the sauce over the chicken.
Finish with the remaining three noodles and the last portion of the spinach cream sauce.
Sprinkle the remaining 1½ cups of mozzarella evenly over the top.
Bake for 25–30 minutes, or until the sauce is bubbling and the cheese is melted and lightly golden.
Remove from the oven and let the lasagna rest for 15–20 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.