In large saucepan, bring 2 quarts water and 2 teaspoons salt to a boil. Cook pasta shells about 9 minutes; shells will be slightly undercooked. Drain and cool.
In large skillet, heat olive oil over medium heat. Add sausage and cook until browned, about 5-6 minutes. Remove sausage from pan with slotted spoon to paper towel-lined bowl. Add garlic to skillet and cook 1 minute or until fragrant. Add spinach. Cook 2-3 minutes, stirring occasionally, until spinach wilts; season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Remove from heat; cool completely.
Preheat oven to 375°F. Prepare 3 quart oval pan with vegetable pan spray.
In large bowl, combine ricotta, 1 cup mozzarella, 1/2 cup parmesan, egg and cooked spinach mixture. Mix until well combined. Stuff cooled shells with mixture. To assemble, spread 1 cup marinara sauce on bottom of pan. Top with half the sausage. Place shells in pan and cover with remaining marinara sauce and sausage. Top with remaining mozzarella and parmesan cheese. Cover pan with aluminum foil.
Bake covered 35-40 minutes or until sauce is bubbly and cheese is melted. Remove foil and turn broiler to high. Place on highest rack and broil 2-3 minutes or until cheese is golden brown. Remove to cooling grid. Serve warm.