Preheat the oven to 180°C.

In a pan, heat olive oil over medium heat. Add chicken, season with salt and pepper, and cook until no longer pink. Set aside.

In the same pan, add the onions, garlic, and mushrooms. Sauté until onions are translucent and mushrooms are cooked.

In a separate pot, melt butter. Add flour and stir until a paste forms. Gradually add milk, stirring continuously to prevent lumps. Cook until sauce thickens.

Combine half the mozzarella and Parmesan cheeses with the sauce. Stir until cheese is melted and incorporated.

Layer the lasagna: Start with a layer of sauce at the bottom of the baking dish, followed by lasagna sheets, then the chicken and mushroom mixture, then another layer of sauce. Repeat these layers.

Top the final layer with the remaining mozzarella and Parmesan cheeses.

Bake in the preheated oven for 45 minutes, or until the top is golden and bubbly.

Allow lasagna to rest for 10 minutes before serving.