Pre-heat your oven to 375 degrees. Lightly mist a 9x13 baking dish with cooking spray and set aside.

Mist a large saute pan or walled skillet with cooking spray and bring to medium-high heat. Add the chicken and onions and cook for 7-10 minutes, breaking up the chicken into bite-sized pieces with a wooden spoon or spatula, until the chicken is cooked through and the onions are softened. Drain if liquid is present. Add the pepper, oregano, salt, nutmeg, red pepper flakes and garlic and stir to combine. Cook for another minute until fragrant. Add the spinach and stir until well combined. Cook for a few additional minutes (if the spinach produces liquid allow it to evaporate) and then transfer to a large mixing bowl. Allow to cool a bit for 8- 10 minutes. Add the beaten eggs, feta and mozzarella cheese and stir together until well combined.

Remove the defrosted phyllo sheets from packaging and place them under a layer of plastic wrap covered by a damp towel to prevent drying. Take out one sheet at a time (a total of 8 sheets) and lay it in the bottom of the prepared baking dish, misting the entire sheet with cooking spray before adding the next sheet until you have a stack of 8 sheets layered in the bottom of the dish. Mist the top phyllo layer and then top with the spinach/chicken mixture. Spread the mixture evenly across the phyllo in the baking dish. Top with another 8 sheets of phyllo dough, misting each layer with cooking spray in between and top with a final mist of cooking spray. Cut the casserole into 8 servings (if you try to do this after baking it will be very messy/flaky).

Place the dish in the oven and bake for 35-45 minute until the top is golden brown.