Preheat your oven to 350°F (175°C). In a large pot, bring salted water to a rolling boil, then add the jumbo pasta shells. Cook for about 10-12 minutes or until al dente, stirring occasionally to prevent sticking. Drain the shells and rinse with cold water to stop the cooking process; set them aside on a clean kitchen towel to cool.

In a large skillet over medium heat, add a drizzle of olive oil. Sauté the diced yellow onion for 3-4 minutes until it becomes translucent, then add minced garlic and cook for an additional minute until fragrant. Next, add the ground beef, cooking until browned and fully cooked, about 5-7 minutes. Mix in the Italian seasoning and half a cup of spaghetti sauce, then let the mixture cool slightly.

In a mixing bowl, combine the ricotta cheese, half of the shredded mozzarella, grated Parmesan, and the large egg. Stir until well mixed, creating a rich and creamy filling. Once the beef mixture has cooled, fold it into the cheese mixture until everything is fully incorporated.

Grease a 9x13 inch baking dish with a little olive oil to prevent sticking. Spread a layer of spaghetti sauce evenly on the bottom of the dish. Carefully fill each cooked pasta shell with the beef and cheese mixture, placing them in the dish side by side. Continue until all the shells are stuffed.

Once assembled, sprinkle the remaining mozzarella and Parmesan cheese generously over the top of the stuffed pasta shells. Cover the dish with aluminum foil to trap moisture. Bake for 25-30 minutes; remove the foil during the last 5-10 minutes.

Once baked, remove the stuffed pasta shells from the oven and let them cool for a few minutes to set. If desired, garnish with fresh herbs or extra cheese. Serve warm.