Start by gathering all your ingredients, and ensure your lasagna noodles are either cooked or ready to use if using the no-boil variety. Chop the onion, garlic, and rosemary, and shred the cooked chicken if not already done.

In a skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic, sautéing until the onions are translucent, about 3-5 minutes.

Add in the shredded chicken and chopped rosemary. Season with salt and pepper to taste. Stir to combine and allow the mixture to heat through for about 2–3 minutes.

In a baking dish, start layering your ingredients: begin with a layer of lasagna noodles, followed by a portion of chicken mixture, béchamel sauce, and a sprinkle of mozzarella and Parmesan cheese. Repeat the layering process until all ingredients are used.

Preheat your oven to 375°F (190°C).

Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 25 minutes.

After 25 minutes, remove the foil and bake for an additional 15 minutes, allowing the top to brown and become bubbly.

Let the lasagna rest for about 10 minutes before cutting to help the layers set.

Slice your Rosemary Chicken Lasagna Layers into squares, serve on plates, and enjoy.