Combine all dumpling ingredients; spoon into a large pastry bag, half at a time, fitted with large plain tube (opening should be about 1/2 in. diameter). Bring 5 qts. of salted water to a boil in a large stock pot, then adjust heat so water bubbles very gently.
Squeeze out dumplings over simmering water, cutting with scissors at 1-in. intervals and letting drop into water. Simmer, uncovered, until dumplings float, then simmer 1 minute more. Remove with a slotted spoon; drain and keep warm in a 2-qt. baking dish.
For the sauce, saute onion in butter until tender. Blend in flour. Add cream and broth all at once. Cook, stirring constantly, until thickened and bubbly.
Remove from the heat; stir in 1/4 cup Swiss cheese and 1/4 cup Parmesan cheese. Pour over dumplings; sprinkle with remaining cheese.
Bake at 350° for 45 minutes or until hot and bubbly and golden brown on top. Sprinkle with parsley if desired.
Editor's Note: Dumplings may also be dropped by teaspoonfuls into boiling water instead of using a pastry bag.