Preheat oven to 375°F (190°C).
Cook jumbo pasta shells in salted boiling water according to package instructions until al dente. Drain and set aside to cool slightly.
Heat a skillet over medium heat and sauté the chopped chicken liver until cooked through, about 5-7 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine the ricotta cheese, half of the mozzarella, half of the parmesan, the egg, cooked chicken liver, and half of the chopped parsley. Mix well until everything is fully incorporated.
Spread 1 cup of pasta sauce evenly in the bottom of a baking dish.
Fill each jumbo shell generously with the cheese and chicken liver mixture and arrange them in the baking dish.
Pour the remaining pasta sauce over the stuffed shells, then sprinkle with the remaining mozzarella and parmesan cheeses.
Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, until the cheese is melted and bubbly.
Garnish with the remaining fresh parsley before serving warm.