1. Cook the pasta shells just shy of al dente in a large pot of salted water. Drain and rinse with cold water to stop the cooking process.
2. In a large skillet, brown the ground beef with diced onion and minced garlic. Drain excess fat and mix in marinara sauce. Let it simmer to create a flavorful meat sauce.
3. In a mixing bowl, combine ricotta cheese, an egg, squeezed-dry thawed spinach, Parmesan cheese, and an Italian cheese blend. Add salt, pepper, and optional pesto for added flavor.
4. Hold each cooked pasta shell carefully and stuff it with the ricotta mixture without tearing the pasta.
5. Preheat the oven to 350°F and spread a layer of meat sauce at the bottom of a 13 x 9-inch baking dish. Arrange the stuffed shells over the meat sauce.
6. Cover the shells with the remaining meat sauce and sprinkle extra cheese on top for a melty, bubbly finish.
7. Cover the dish with aluminum foil and bake for 30 to 40 minutes. Remove the foil and bake for an additional 5 to 10 minutes until the cheese is golden and bubbly.
8. Let the dish cool slightly before serving. Garnish with basil or parsley for a fresh touch.
9. To store, allow the dish to cool before refrigerating in an airtight container for up to 3-4 days or freeze for up to 3 months. Reheat as needed.