1. In large nonstick skillet, cook sausage over medium-high heat, breaking up meat with wooden spoon, until meat is no longer pink, 5 minutes. Add onion; cook, stirring occasionally, until onion is softened, about 5 minutes. Add garlic; cook 1 minute. Add marinara sauce; simmer over medium heat for 10 minutes or until sauce begins to thicken. Stir in pesto. Set aside.
2. Cook penne following package directions.
3. Heat oven to 375 degrees F. Coat 13 x 9 x 2-inch baking dish with nonstick cooking spray. Spread thin layer of sauce over bottom of dish.
4. In large bowl, combine drained cooked penne, remaining sauce, cubed cheese, 1/3 cup Parmesan and the spinach. Spoon pasta mixture into baking dish. Sprinkle remaining Parmesan over top.
5. Bake in 375 degree F oven for 30 minutes or until bubbly.