Preheat your oven to 375°F.
Cook the lasagna noodles in a large pot of salted water until they reach al dente, referring to the package instructions. Drain them and set aside to cool slightly.
In a 4-quart saucepan, melt butter over medium heat. Once melted, add flour, black pepper, sea salt, garlic powder, and onion powder. Stir the mixture constantly and cook for 1 to 2 minutes until it turns a light golden brown and becomes bubbly.
Slowly whisk in whole milk, ensuring it blends smoothly with the roux. Continue cooking over medium heat until the mixture reaches a low boil and begins to thicken, about 6 to 8 minutes.
Add cream cheese and ½ cup of shredded jack cheese to the white sauce. Stir until the cheese melts and the sauce is creamy and well-combined. Once done, remove from the heat and set aside.
Gently stir the baby spinach leaves into the white sauce, allowing them to wilt slightly from the heat.
In a small mixing bowl, combine ricotta, the lightly beaten egg, and another ½ cup of jack cheese.
In a greased 9×13-inch glass baking dish, spread a bit of the white sauce on the bottom to prevent sticking. Then, lay three lasagna noodles side by side in the dish.
Spread half of the ricotta mixture over the noodles, then top with half of the diced chicken. Spoon a third of the white sauce over this layer, followed by a cup each of mozzarella and jack cheese.
Repeat the layering process: add another layer of noodles, the remaining ricotta, the remaining chicken, and another cup each of mozzarella and jack cheese.
Top with the last layer of noodles, followed by the remaining white sauce and all the remaining cheese. Make sure everything is covered nicely.
Place the dish in the oven and bake for 30 to 35 minutes, or until the sauce is bubbling and the cheese is melted and starting to brown.
Once done, remove the lasagna from the oven. Let it sit for a few minutes before slicing. Garnish with more parsley if desired.
