Preheat your oven to 375°F (190°C). In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, which should take about 3-4 minutes.
Add the sliced mushrooms to the skillet and cook them until they release their juices and begin to brown, approximately 5-7 minutes.
Stir in the chopped spinach and cook until it is just wilted, then take the skillet off the heat.
In a large bowl, mix together the shredded chicken, cottage cheese, mushroom mixture, and half of the shredded mozzarella. Season the mixture generously with salt and pepper.
Transfer the mixture into a baking dish, spreading it out evenly. Sprinkle the remaining mozzarella on top.
Bake in the preheated oven for 20 to 25 minutes, or until the casserole is bubbling and lightly golden on the surface.
Allow it to rest for about 5 minutes before serving. Enjoy it warm with a crisp salad on the side.
