Step 1: Bring a large pot of generously salted water to a boil. Add the jumbo pasta shells and cook according to package directions until al dente. Drain thoroughly and set aside.
Step 2: In a large skillet over medium heat, brown the ground beef. Add the diced onion and minced garlic, cooking until the beef is fully cooked and the onion softens (5-7 minutes). Drain any excess fat. Stir in the tomato sauce, Italian seasoning, salt, and pepper. Simmer gently for 5 minutes.
Step 3: Preheat your oven to 350°F (175°C). In a separate bowl, combine the ricotta cheese with half of the mozzarella and half of the Parmesan. Fill each cooked pasta shell with a generous spoonful of the seasoned beef mixture. Arrange the stuffed shells snugly in a greased 9x13-inch baking dish. Pour any leftover beef sauce over the shells.
Step 4: Sprinkle the remaining mozzarella and Parmesan cheese over the assembled casserole. Cover the baking dish tightly with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 10-15 minutes, or until the cheese is beautifully melted, bubbly, and golden brown.
Step 5: Once baked, remove the casserole from the oven and let it rest for 5-10 minutes. Garnish with fresh chopped parsley, if desired. Serve warm.