Preheat oven to 400 degrees F. Bring a large pot of lightly salted water to a boil. Spray a 9 x 13” baking dish with cooking spray.
Cook pasta according to package directions. Drain and set aside.
While pasta cooks, spray a large skillet with cooking spray, and heat it over medium-high heat. Add sausage, onion and mushrooms to the hot skillet. Cook for 8-10 minutes, stirring and breaking up meat frequently.
Add pasta sauce to the sausage, and bring to a simmer. Stir in spinach, and cook until wilted, 3-5 minutes.
In a large bowl (or the pasta cooking pot), stir together pasta and sauce mixture. Transfer pasta to prepared baking dish. Spread mozzarella over the pasta, and arrange pepperoni slices in an even layer over the cheese.
Bake in preheated oven for 10 minutes, or until cheese is melted and pepperoni are starting to brown.