Preheat oven to 190 °C (375 °F).
In a large pot of salted boiling water, cook the pasta al dente. Drain.
In a bowl, combine the shredded duck with the ricotta, spinach and, if desired, green onions.
Stuff the shells with the duck confit mixture.
Spread some of the marinara sauce over the bottom of a 33 cm x 23 cm (13 in. X 9 in.) baking dish. Lay the shells side by side in the dish. Cover the shells with the rest of the marinara sauce.
Bake in the oven for 25 to 30 minutes.