In a large skillet over medium-high heat, sauté onion in butter until onion is translucent. Stir in drained spinach and cook until almost dry, about 5 minutes. Add ricotta cheese and cook an additional 2 minutes. Remove from heat and cool to room temperature.
Stir in flour, ½ cup parmesan cheese, salt, pepper, garlic powder, nutmeg, parsley, and eggs.
Shape into walnut-sized balls, using additional flour as needed. (If not cooking immediately, coat with flour and store in a single layer in a covered glass dish in the refrigerator for up to 24 hours.)
Heat water until just boiling. Drop about 12 dumplings at a time into boiling water; cook 5-6 minutes or until dumplings float to the surface. Remove from water with slotted spoon and place in a 9x13-inch glass baking dish and keep warm. Repeat until all dumplings are cooked.
Drizzle the ½ cup melted butter over the cooked dumplings. Sprinkle with the remaining parmesan cheese. Serve hot.