Preheat oven to 350 degrees F (175 degrees C). Grease a baking dish with cooking spray.
Spread sweet potatoes evenly over the bottom of the prepared dish.
Heat a large saucepan over medium heat. Add turkey sausage and spinach; cook and stir until turkey is browned and spinach is wilted, about 5 minutes. Remove from heat. Add to sweet potatoes.
Crack egg whites into a large bowl; add almond milk, sage, red pepper flakes, thyme, oregano, basil, nutmeg, salt, and pepper. Stir well. Pour egg mixture over the top of the casserole. Sprinkle with Parmesan cheese.
Bake in the preheated oven until set, about 45 minutes. Cut casserole into 6 pieces.