01 - Preheat oven to 375°F. Lightly grease a 9x13 inch baking dish.
02 - Boil jumbo pasta shells in salted water as per package directions until al dente. Drain the shells and set aside.
03 - Combine ground beef, egg, breadcrumbs, Parmesan, parsley, garlic, oregano, salt, pepper, and milk in a large mixing bowl. Mix gently until just combined.
04 - Shape the mixture into 24 small meatballs, approximately 1 inch in diameter.
05 - Heat a large skillet over medium heat and add olive oil. Brown the meatballs on all sides for 5 to 7 minutes. Meatballs do not need to be fully cooked through.
06 - Pour about 1 cup marinara sauce onto the bottom of the prepared baking dish to coat evenly.
07 - Fill each cooked pasta shell with a meatball and arrange stuffed shells, open side up, in the baking dish.
08 - Pour remaining marinara sauce evenly over the shells. Sprinkle shredded mozzarella and grated Parmesan on top.
09 - Cover dish with foil and bake for 25 minutes.
10 - Remove foil and bake for an additional 10 to 15 minutes, until cheese is melted and bubbly.
11 - Garnish with fresh basil leaves before serving, if desired.