Heat the oven to 375°F and lightly grease a 9-by-13-inch baking dish.

Cook the jumbo shells in salted boiling water until just shy of tender, usually 1 to 2 minutes less than the package directions. Drain and rinse briefly with cool water, then keep them out of the water so they do not become soggy.

Brown the Italian sausage in a large skillet over medium-high heat. Add the chopped onion and cook for 3 to 4 minutes, then stir in the garlic and cook for about 30 seconds; drain excess fat if needed.

Mix the ricotta, egg, Parmesan, Italian seasoning, kosher salt, and black pepper in a bowl. Stir in approximately half of the cooked sausage mixture.

Spread about 1 cup of marinara in the prepared dish. Fill each shell with roughly 2 tablespoons of the ricotta-sausage filling and arrange the shells filling side up.

Spoon the remaining marinara over the stuffed shells and sprinkle with 1 1/2 to 2 cups of shredded mozzarella.

Cover the dish with foil and bake for 15 minutes. Remove the foil and bake for another 5 minutes, or until the cheese is melted and bubbling.

Rest the casserole briefly, then garnish with chopped parsley or basil before serving.