1. In large pot of boiling salted water, cook the penne until just al dente. Drain and save 1 cup of the cooking water.

2. Meanwhile, in large, deep skillet, heat the olive oil until shimmering. Add the onion and cook over medium-high heat, stirring, until softened, 4 to 5 minutes. Add the sausage and crushed red pepper. Break up the meat with the back of spoon as it cooks, until no trace of pink remains, about 5 minutes. Add the spinach and tomatoes and cook just until softened, about 3 minutes.

3. Add the penne and the reserved cooking water to the skillet and cook over medium-high heat, lightly crushing the tomatoes, until heated through, about 2 minutes, season with salt. Transfer the pasta to bowls, sprinkle with Parmesan and serve.