In a large bowl, combine the shredded chicken with ricotta cheese, 1 cup of mozzarella, ½ cup of Parmesan, chopped parsley, garlic powder, salt, and pepper. Stir until everything is evenly mixed. The ricotta adds creaminess while the mozzarella brings the melt. Set the mixture aside while you prepare the sauce.
In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for about a minute or two to eliminate the raw flour taste. Gradually whisk in the milk, stirring constantly to avoid lumps. Let it simmer until thickened. Stir in garlic powder, Parmesan, salt, and pepper, and turn off the heat.
Preheat your oven to 375°F (190°C). In a greased 9x13 inch baking dish, start by spreading a thin layer of the white sauce. Place lasagna noodles over the sauce, then spoon on some of the chicken filling. Drizzle more white sauce (and marinara if using). Repeat these layers until all ingredients are used, finishing with noodles and white sauce on top.
Sprinkle the remaining mozzarella and Parmesan cheese over the top layer. This cheese topping creates a beautiful golden crust. Cover loosely with foil and bake for 25–30 minutes or until bubbly and the top is golden brown. Remove foil for the last 10 minutes if you want more browning.
Let the lasagna cool for about 10 minutes before slicing. This allows the layers to set and makes serving easier. Garnish with freshly chopped parsley for a pop of color and flavor. Serve warm, and enjoy the creamy, cheesy goodness.