Preheat oven to 375°F.
Cook the lasagna noodles in salted water until they reach al dente (follow the package instructions to cook). Drain and set aside.
In a 4-quart saucepan, melt butter over medium heat. Add the flour, pepper, salt, garlic powder, and onion powder to the melted butter and cook for 1 to 2 minutes until the mixture is browned and bubbly.
Whisk the mixture while slowly pouring in milk until smooth and combined. Continue cooking over medium heat until the mixture reaches a low boil and begins to thicken (6 to 8 minutes).
Add cream cheese and ½ cup shredded jack cheese to the white sauce. Stir, still cooking over medium heat, until cheese is melted and sauce is thoroughly combined. Remove from the heat.
Stir the spinach leaves into the white sauce and set aside.
In a small mixing bowl, combine the ricotta cheese, egg, parsley, and another ½ cup of jack cheese.
In a greased 9×13-inch glass baking dish, spoon a bit of the sauce to cover the bottom of the pan. Place three lasagna noodles side by side into the pan.
Spread ½ of the ricotta mixture over the noodles and top with ½ of the chicken. Spoon ⅓ of the sauce over the chicken and ricotta and top with 1 cup each of mozzarella and jack cheese.
Place another layer of noodles, the remaining ricotta, the remaining chicken, and another cup of both mozzarella and jack cheese.
Top with the remaining noodles, the remaining sauce, and the remaining cheese.
Bake lasagna for 30 to 35 minutes or until the sauce is bubbling up and the cheese is melted and beginning to brown.
Remove from the oven and garnish with more parsley, if desired.