Preheat your oven to 375°F (190°C) to ensure even baking.

Heat a splash of olive oil in a large skillet over medium heat. Add minced garlic and fresh rosemary, sautéing for 2-3 minutes until fragrant.

In a saucepan, melt unsalted butter, whisk in flour for 1-2 minutes, then gradually add whole milk. Whisk until the mixture thickens, about 5-7 minutes. Season with nutmeg and add grated Parmesan.

In a mixing bowl, combine shredded chicken and baby spinach with the béchamel sauce.

In a baking dish, layer a thin layer of béchamel, three lasagna noodles, one-third of the filling, one-quarter of the béchamel, and shredded mozzarella. Repeat the layers until all are used.

Cover with foil and bake for 30 minutes, then uncover and bake for an additional 10-15 minutes until bubbly and golden brown.

Let the lasagna rest for 10 minutes before slicing to allow the layers to set.