How to Make It
In a large pot of boiling salted water, cook the gemelli until just al dente. Drain the gemelli, reserving 1 cup of the cooking water.
Meanwhile, in a large, deep skillet, heat the olive oil until shimmering. Add the onion and cook over moderately high heat, stirring, until softened, 4 to 5 minutes. Add the sausage and crushed red pepper and cook, breaking up the meat with the back of a spoon, until no trace of pink remains, about 5 minutes. Add the spinach and tomatoes and cook just until softened, about 3 minutes.
Add the gemelli and the reserved cooking water to the skillet and cook over moderate heat, lightly crushing the tomatoes, until heated through, about 2 minutes; season with salt. Transfer the pasta to bowls, sprinkle with the Parmesan and serve.