To make the filling, mash ricotta, parsley, Parmesan, nutmeg, lemon zest, salt and pepper together in a bowl until well blended. Cover and chill while preparing dough.
Follow the instructions above for rolling out the pasta into wide sheets. The resulting sheet of pasta should be about 1/16 inch thick—just short of being translucent.
Use a pastry wheel or 4-to 5-inch cookie cutter to cut pasta into squares. Spoon 1 rounded teaspoonful of filling into center of each dough square. Brush edges lightly with egg white; fold over into a triangle and press to seal. Fold and press tips together, moistening with egg white to seal. Transfer to a lightly floured rimmed baking sheet. The pasta can be made up to 6 hours ahead—cover with plastic wrap or a kitchen towel and chill until ready to cook.
Bring stock to a simmer in a large saucepan. Add tortelloni to boiling water. Simmer, stirring gently, until cooked through—they will begin to float after 2 to 3 minutes, alerting you that they are cooked. Using a slotted spoon, transfer tortelloni to serving bowls and spoon sauce over them. Serve, sprinkled with cheese and chives.