Season the chicken with salt, pepper and lemon juice. In a frying pan, heat a tablespoon of olive oil over medium heat and cook the chicken for 5-6 minutes, stirring occasionally to cook evenly. Remove and set aside.
Thoroughly clean and slice the mushrooms. In a frying pan, which can be the same where you cooked the chicken, heat a tablespoon of olive oil with the garlic cloves and rosemary over medium-high heat. Add the mushrooms and let gain some color evenly, as they lose some of their natural moisture. Season with salt and freshly grounded black pepper. Remove the garlic and rosemary, add the chicken and mix to combine.
Preheat the oven to 160°C (the temperature and cooking time can vary with the type or brand of pasta that you are using. Check the package for precise instructions).
To make the white sauce, combine the olive oil and the flour in a small pan and cook for 1 to 2 minutes over low heat, stirring constantly and without allowing the flour to gain color. Add the milk, season with salt and freshly grounded black pepper and nutmeg and let the milk come to a boil and the sauce thickens. Stir continuously with a wire whisk to avoid the formation of lumps.
To assemble the lasagna, pour some white sauce on the bottom of the pan that will go into the oven. Arrange a layer of lasagna sheets, and then half of the chicken and mushroom mixture. Crumble half the mozzarella cheese on top and cover with one third of the white sauce. Cover with a new layer of lasagna sheets and repeat the process. Top with the last layer of pasta, cover with the remaining white sauce and sprinkle generously with freshly grated Parmesan cheese. Garnish with rosemary sprigs.
Bake in the preheated oven for 25 minutes. During the last 10 minutes turn the grill function on for a more beautiful golden color.
Wait 10 minutes to set a little and serve with a green salad.