Prepare the béchamel: Melt the butter in a saucepan, add the flour and mix well. Pour the boiling milk, stirring constantly to avoid lumps, and bring to boil. Season with salt, a pinch of nutmeg and set aside.
Peel the onion and the carrots. Wash the celery.
Cut the vegetables into small chunks and brown then cook them slowly in a saucepan with the olive oil and a bunch of rosemary. Add the chicken meat and let it brown as well.
Add the tomato paste, cover with water and let it cook for 10 minutes.
Season with salt and pepper. Remove the rosemary and set aside.
Grease a baking pan with butter and pour a first layer of chicken sauce and béchamel at the bottom. Add a few drops of pesto. Then add a layer of lasagne, followed by another layer of sauces.
Sprinkle with grated Parmigiano Reggiano and continue to build the lasagna, alternating the pasta and sauces.
Make sure the final layer is sauce, and sprinkle with Parmigiano Reggiano. Bake in oven at 180° for 20 minutes.
Remove from the oven when it’s perfectly baked, crispy and golden brown. Let it cool for about 10 minutes before serving.