Gather all your ingredients. Shred the cooked chicken breast and finely chop the fresh rosemary.

In a skillet over medium heat, warm the olive oil. Add minced garlic and fresh rosemary, sautéing for about a minute until fragrant.

Stir in the shredded chicken, salt, and black pepper. Cook for another 2-3 minutes, then remove from heat.

In a saucepan over medium heat, melt the unsalted butter.

Whisk in the flour and cook for about a minute, then gradually pour in the whole milk while whisking to avoid lumps.

Continue stirring until the sauce is thick and creamy, for about 5 to 7 minutes. Season with nutmeg and stir in the grated Parmesan cheese, then remove from heat.

Spread a thin layer of béchamel sauce on the bottom of a 9x13-inch baking dish.

Layer three lasagna noodles over the sauce, followed by one-third of the chicken mixture and a quarter of the remaining béchamel sauce. Top it with a quarter of the shredded mozzarella.

Repeat this process two more times to create three layers.

For the final layer, place the remaining lasagna noodles on top. Spread the remaining béchamel sauce evenly.

Sprinkle the remaining mozzarella and half a cup of grated Parmesan cheese on top.

Cover the dish with foil and bake in a preheated oven at 375°F for 30 minutes.

Remove the foil and bake for an additional 10-15 minutes until golden brown and bubbly.

Allow the lasagna to rest for about 10 minutes before slicing.

Serve warm and garnish with fresh rosemary or additional Parmesan as desired.