Bring a large pot of lightly salted water to a boil. Add shells and cook for 8 to 10 minutes or until al dente; drain and rinse in cold water.
Place sausage in a large, deep skillet. Cook over medium-high heat until evenly brown. Drain and crumble. In a large bowl, combine cooked sausage, spinach, ricotta cheese, egg, garlic, lemon juice and Parmesan cheese. Season with salt, pepper and oregano.
Preheat oven to 350 degrees F (175 degrees C).
Stuff pasta shells with the sausage and cheese mixture and place in a 9x13 inch baking dish. Top with spaghetti sauce and mozzarella cheese.
Bake in preheated oven for 20 minutes or until the pasta is heated through and the cheese is melted.