Gather all ingredients and preheat the oven to 350°F (175°C).

Bring a large pot of lightly salted water to a boil. Add lasagna noodles and cook for 8 to 10 minutes or until al dente; drain and rinse with cold water.

Melt butter in a large saucepan over medium heat; stir in onion and garlic. Whisk in flour until lightly browned and onion is tender, about 2 minutes.

Add chicken broth, milk, and salt; cook, whisking continuously, for 1 minute.

Stir in 2 cups of mozzarella and 1/4 cup Parmesan until well combined. Season with basil, oregano, and black pepper; remove from heat and set aside.

Spread 1/3 of the sauce mixture in the bottom of a 9x13-inch baking dish. Add a layer with 3 lasagna noodles, ricotta, and chicken.

Arrange 3 lasagna noodles over the chicken and layer with 1/3 of the sauce mixture, spinach, remaining 2 cups mozzarella, and 1/2 cup Parmesan.

Arrange remaining noodles over cheese and spread remaining sauce evenly over noodles. Sprinkle with parsley and remaining 1/4 cup Parmesan.

Bake in the preheated oven for 35 to 40 minutes.