Preheat the oven to 350°F.
Bring a large pot of salted water to a boil. Add the shells and cook until al dente, about 9 minutes. Drain the shells, rinse them with cool water, then set them aside.
Add the ground beef to a large nonstick skillet set over medium heat and cook, breaking it apart with a spatula, until it is no longer pink. Using a slotted spoon, transfer the ground beef to a bowl and set it aside to cool completely.
Once the ground beef has cooled, stir in the ricotta cheese, spinach, egg, oregano, 1/2 teaspoon salt and 1/4 teaspoon pepper.
Add half of the tomato sauce to the bottom of a 10-inch skillet or baking dish.
Divide the ground beef mixture among the shells then arrange them in a single layer in the baking dish. Top the shells with the remaining sauce then sprinkle them with the mozzarella cheese.
Bake the shells until the cheese is melted and the shells just begin to crisp, about 25 minutes.
Top the shells with grated Parmesan cheese and fresh basil and then serve.