Cook jumbo pasta shells until just al dente, drain them, rinse with cold water, and cool on a sheet pan.
Sauté ground beef with diced onion and minced garlic, season with Italian herbs, salt, and pepper.
In a large bowl, fold together ricotta, shredded mozzarella, grated Parmesan, beaten egg, parsley, and the cooled beef mixture.
Generously stuff each shell with the seasoned cheese–beef mixture and arrange them open-side up in the baking dish.
Pour remaining sauce over the shells, top with extra cheese, cover with foil, bake until bubbly, then uncover, brown, and let rest before serving.