These delicious little balls of spinach and cheese are similar to gnocchi. Be sure to cook most of the moisture out of the spinach so the balls hold together. Test by cooking a couple of dumplings. If the mixture is loose, add another tablespoonful of flour.
Melt butter in a frying pan or skillet. Add onion and cook until translucent. Add spinach and cook, stirring occasionally, until moisture is evaporated. Add ricotta, remove from heat, and stir until well mixed, about 3 minutes. Pour into bowl. Add eggs, cheese, salt, pepper, and nutmeg. Stir well. Mix in flour, cover, and chill in refrigerator at least 1 hour, or overnight. (At this point, dumplings can be frozen.) With floured hands, roll mixture into balls the size of cherry tomatoes. Place on waxed paper. Meanwhile, bring water to a boil in a large pot. Cook dumplings in boiling water 6 to 8 minutes (they should rise to the top). Remove with slotted spoon and allow them to rest a few minutes before serving. Serve with red sauce or with butter and Parmesan cheese. For meatless meal, serve with marinara sauce.