Heat the olive oil in a large skillet over medium heat.
When the oil is shimmering, add the ground beef, 1 teaspoon of salt, ½ teaspoon of garlic powder, ½ teaspoon of Italian seasoning, and ⅛ teaspoon of pepper.
Cook, stirring, until the meat is no longer pink.
Stir in the tomato sauce and bring the mixture up to a simmer.
Cook the pasta shells according to the directions on the package.
In a large mixing bowl, combine the ricotta, half the mozzarella, the Parmigiano, egg, the remaining ½ teaspoon of salt, the remaining ½ teaspoon of garlic powder, the remaining ⅛ teaspoon of pepper, and the parsley, folding together until blended.
Coat the bottom of a 2-quart baking dish with about ⅔ of the meat sauce.
Fill each pasta shell with about 2 to 3 tablespoons of the filling, then nestle the stuffed shells over the sauce and into the baking dish.
Top with the remaining meat sauce and mozzarella cheese.
Cover with foil and bake in a preheated 375 degree F oven until bubbling (approx. 30 minutes), then uncover and allow the cheese to brown (about 10 minutes more).
Garnish with Parmigiana, if desired.