Preheat oven to 375F. Spray a 9x13 inch baking dish with cooking spray.
Mix the ground turkey, fennel, garlic powder, and salt and pepper. Add to a large skillet set over medium-high heat. Cook, stirring frequently until no longer pink. Transfer to a bowl.
Add oil to the skillet. When the oil is hot, add the onion and garlic. Cook, stirring, until the onion is soft - about 5 minutes. Add the mushrooms and cook another 5 minutes. Add the spinach and cook, stirring, until wilted - about 2 minutes. Stir in the turkey.
Spread 1 cup of sauce in the bottom of the prepared baking dish. Cover with 4 lasagna noodles, overlapping as needed. Layer half of the filling on top of the noodle, then layer with half the ricotta. Top with 1 cup of sauce, half the pepperoni, and a sprinkle of red pepper flakes. Add another layer of noodles and repeat the filling, ricotta, and sauce layers. Top with remaining noodles and sauce. Sprinkle with mozzarella and basil.
Cover with foil and bake 30 minutes. Remove foil and bake 20 minutes, or until noodles are soft and mozzarella has browned. Remove from oven and let sit for 10 minutes before slicing.