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.

Cook sausage in a large, deep skillet over medium-high heat until evenly brown. Drain and crumble. Combine cooked sausage, spinach, ricotta cheese, egg, garlic, lemon juice, and Parmesan cheese in a large bowl. 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 until the pasta is heated through and the cheese is melted, about 20 minutes.