Bring a large pot of salted water to a boil.
Heat the oil in a large sauté pan over medium-low heat.
Add the onion and sauté until it is tender and begins to color, 10 minutes.
Crumble the sausage into the pan, increase the heat to medium-high, and fully cook the sausage.
Cook the rigatoni until just shy of al dente.
Before draining, reserve about 1 cup of the cooking water.
Add the spinach and tomatoes and some of the pasta cooking liquid to the sausage and cook, tossing, until the spinach has wilted.
Add the pasta, half of the reserved cooking liquid, Parmigiano-Reggiano, and goat cheese.
Toss well and lightly smash the tomatoes.
Add a bit more of the cooking liquid if needed and serve sprinkled with parsley, if you like.