Preheat your oven to 350°F. Bring a large pot of salted water to a boil, then add the jumbo pasta shells. Cook them just until al dente, being careful not to overcook, as overcooked shells are more likely to tear when stuffed. Drain the shells in a colander, rinse briefly with cool water, and set aside.

Heat a large skillet over medium heat and add the cooking oil. Brown the ground beef chuck, then drain any excess fat. Once browned, add the diced onion and minced garlic to the skillet. Sauté over medium heat until the onion is translucent and the garlic becomes fragrant.

Next, stir in the marinara sauce, tomato paste, Italian seasoning, salt, garlic powder, and freshly cracked black pepper. Reduce the heat to medium-low and let the beef sauce simmer for 3 to 5 minutes.

While the sauce simmers, prepare the cheese filling. In a medium bowl, combine the ricotta, shredded mozzarella, grated Parmesan, fresh chopped parsley, salt, black pepper, and egg. Mix well until fully incorporated.

Spread half of the beef sauce evenly over the bottom of a large 9×13-inch casserole dish.

Stuff each shell with 1 to 2 tablespoons of the cheese filling—about one heaping tablespoon per shell works well. Place the stuffed shells upright in the casserole dish as you go.

Once all the shells are stuffed and arranged, pour the remaining beef sauce over the top. Sprinkle 1½ cups of shredded mozzarella cheese evenly over everything.

Loosely cover the casserole dish with tented aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and continue baking uncovered for an additional 5 to 10 minutes, or until the sauce bubbles around the edges and the cheese is melted and slightly golden. Serve hot and enjoy!