Prepare the Meat Sauce: Heat a tablespoon of oil in a large skillet over medium heat. Sauté the chopped onion and minced garlic for about 2-3 minutes, until the onion is translucent. Add in the ground beef, oregano, black pepper, and salt. Cook for approximately 5-7 minutes until browned. Pour in the marinara sauce, cover, and gently simmer for 10 minutes.

Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the large pasta shells and cook for about 5-6 minutes, just until slightly undercooked. Drain and rinse under cold water briefly. Set aside to cool.

Mix the Cheese Filling: In a medium-sized bowl, combine the grated Parmesan cheese, cream, and grated mozzarella. Mix until smooth.

Assemble the Stuffed Shells: Preheat your oven to 320°F (160°C). Grease an ovenproof tray. Spread a layer of meat sauce on the bottom, fill each shell with the cheese mixture, place in the tray seam side up, cover with remaining sauce and sprinkle more mozzarella on top.

Bake to Perfection: Transfer the assembled stuffed shells into the oven and bake for about 20 minutes, or until the cheese is bubbly and golden brown.

Serve Warm: Allow to sit for a few minutes before serving to let flavors meld.