Heat the olive oil in a 12-inch cast-iron skillet over medium-high heat. Add the ground beef, breaking it into hearty chunks. Sizzle until a deep brown crust forms, then drain excess fat and lower the heat to medium.

Stir in the diced onion and minced garlic, sautéing until translucent. Pour in the crushed tomatoes, dried Italian herbs, salt, and pepper. Simmer gently on low for 15 minutes until thick and chunky. Set half the sauce aside.

Bring a large pot of heavily salted water to a rolling boil. Cook the manicotti shells for exactly 2 minutes less than package instructions for a firm al dente texture. Drain carefully and cool on parchment paper.

In a mixing bowl, vigorously whisk the whole milk ricotta, egg, grated Parmesan, salt, and black pepper for 60 seconds until light and fluffy. Transfer to a piping bag.

Preheat oven to 375°F (190°C). Insert the piping bag tip into each cooled manicotti shell and squeeze until the fluffy ricotta mixture is flush with both ends.

Arrange the stuffed pasta side-by-side in the cast-iron skillet directly over the remaining bottom layer of sauce. Spoon the reserved chunky meat sauce generously over the top of the pasta.

Cover tightly with foil and bake for 25 minutes. Uncover, sprinkle with shredded mozzarella, and bake for 10 more minutes. Switch oven to broil for 2 minutes until the cheese is bubbly and golden-brown. Garnish with fresh chopped parsley and let rest 10 minutes before serving.