Bring a large Dutch oven of salted water to a boil. Meanwhile, in a large, ovenproof skillet, melt the butter over medium-low heat. Add the onion and cook, stirring occasionally, until softened and golden, 10 to 12 minutes. Add 1 tablespoon flour and cook, stirring constantly, 2 minutes. Add the half-and-half and simmer, stirring constantly, until slightly thickened, about 3 minutes. Add the nutmeg. Season with salt and pepper. Working in batches, stir in the spinach and cook over low heat until just wilted, 3 to 5 minutes. Cover and set aside.
Preheat the broiler. In a medium bowl, lightly beat together the ricotta, eggs, 1/2 cup parmesan, the remaining 1 1/2 cups flour, 1 teaspoon salt and 1/2 teaspoon pepper until well combined. Using a soupspoon, drop 10 to 12 generous spoonfuls of dough into the boiling water and cook for 3 minutes; gently stir the gnocchi. When the gnocchi rise to the surface, cook for about 3 minutes more. Using a slotted spoon, transfer to a paper-towel-lined plate to drain. Repeat with the remaining batter.
Add the gnocchi to the spinach sauce and stir gently to coat. Sprinkle with the remaining 1/2 cup parmesan and broil until golden, about 3 minutes.