Bring a large pot of salted water to a boil. Remove the stems from the spinach. Swish the spinach leaves in a large bowl or sinkful of cool water to clean. If there is a lot of sand and grit in the bottom of the bowl, change the water and wash the leaves again. Drain the spinach leaves and dry them well, preferably in a salad spinner. Stack several of the leaves and cut them crosswise into 1/4 inch strips. Repeat with the remaining spinach. You should have about 8 packed cups of shredded spinach. Stir the fusilli into the boiling water. Cover the pot and reheat to boiling, stirring once or twice. When the water returns to a boil, uncover the pot. Cook the pasta, stirring occasionally, until al dente— tender but firm— about 12 minutes. Meanwhile, in a large skillet heat the olive oil over medium heat. Add the onions and cook until wilted, about 3 minutes. Stir in the spinach and a pinch each of salt and pepper. Cover the skillet and steam the spinach until wilted but still bright green, about 3 minutes. In a small bowl stir the ricotta and half and half until smooth. Stir the ricotta mixture and the butter into the skillet. Add salt and pepper to taste. Reduce the heat and simmer 5 minutes. Reserve ½ cup of the pasta cooking liquid. Drain the pasta well and return it to the pot over low heat. Add the spinach mixture and enough of the reserved cooking liquid to make a sauce that lightly coats the pasta. Toss thoroughly. Remove the pot from the heat and stir in the grated cheese. Transfer the pasta to a warm serving platter or individual bowls and serve immediately.