Boil salted water in a large pot, add pasta shells, cook until al dente, about 8-10 minutes. Drain, rinse with cold water, and toss with olive oil.

In a mixing bowl, mix cooked ground beef with ricotta, mozzarella, Parmesan, salt, and pepper until creamy.

Fill each pasta shell with the beef and cheese mixture and arrange in a greased baking dish.

Spread marinara sauce at the bottom of the baking dish, add stuffed shells, pour remaining sauce over, and sprinkle additional mozzarella.

Preheat oven to 350°F (175°C), cover the dish with foil, and bake for 30 minutes. Uncover and bake an additional 10 minutes until bubbly.

Remove from oven, let rest for a few minutes, garnish with parsley, and serve warm.