Cook couscous: pour 1 ½ cups of water into a saucepan and bring to a boil over high heat. Pour couscous into the saucepan. Stir well. Cover and let stand for 5 minutes, or until all the liquid is absorbed. Fluff with a fork.
Heat oven to 350°F. Lightly grease a 2-quart baking dish with olive oil. Set aside.
Add oil to a large pan or skillet and saute onions and mushrooms over medium heat for 5-8 minutes, or until softened. Add tomato paste and mix thoroughly. Next, stir in spinach and chicken and cook for about 2-3 minutes until spinach just starts to wilt.
Evenly spread the couscous in the prepared baking dish. Spread sauce over the couscous. Make a layer of the vegetables and chicken. Finally spread 1 cup of mozzarella on top. If preparing ahead, stop here and wrap the dish with plastic wrap and then foil.
Bake, covered, for about 20-25 minutes.
Storage tips: Transfer to an airtight container and store in the fridge for up to 4 days or in the freezer for up to 3 months.For a vegetarian version, you can swap out chicken with tofu or beans.