Set the oven to 400 degrees and lightly oil an 8 x 8-inch baking dish. The dish should be ready before you start layering so the noodles and sauce go in while everything is still easy to handle.

Melt the butter in a saucepan over medium heat, then stir in the garlic and flour and cook for 30 seconds. This short cook takes away the raw flour taste while keeping the roux pale and smooth.

Whisk in the milk gradually and bring the sauce to a gentle simmer until it thickens. It should be thick enough to coat the back of a spoon, which helps the layers hold together instead of sliding apart.

Take the pan off the heat, then stir in the salt, Parmesan, cream cheese, and spinach until the cheese melts and the spinach wilts. The finished sauce should look smooth, creamy, and evenly flecked with green.

Whisk the ricotta and egg together in a small bowl until fully blended. The mixture should look creamy and uniform, with no separate streaks of egg.

Spread about 1/4 cup sauce in the baking dish, then arrange 2 noodles on top. That thin base layer helps keep the noodles from sticking to the bottom.

Add ¼ cup of the ricotta mixture, 2/3 cup chicken, 2/3 cup mozzarella, 1 tablespoon Parmesan, and ½ cup sauce over the noodles. Try to spread each layer evenly so every slice gets the same balance of filling and sauce.

Repeat the same layering pattern twice more, then finish with the remaining sauce and Parmesan over the top layer of noodles. The top should be well covered so the pasta stays tender as it bakes.

Bake uncovered for 20 to 25 minutes, until the sauce is bubbling. The edges should look hot and lively, and the top should be melted and lightly golden in spots.

Let the lasagna cool for 15 to 20 minutes before serving. This rest is crucial because it allows the layers to set for cleaner, prettier slices.