In a small saucepan, combine tomatoes (with their juice) and 3 cups water. Bring to a simmer; keep warm over low heat.
In a medium saucepan, heat oil over medium heat. Add sausage and onion; season with salt and pepper. Cook, breaking up sausage until sausage is cooked and onion has softened, about 3-5 minutes.
Add rice; cook, stirring until well coated, 1 to 2 minutes. Add wine or broth; cook, stirring until absorbed about 1 minute.
Add about 2 cups of the hot tomato mixture to rice; simmer over medium-low heat, stirring occasionally, until absorbed, 4 to 5 minutes. Continue adding tomato mixture, 1 cup at a time, waiting for it to be absorbed before adding the next, stirring occasionally, until rice is creamy and just tender, about 25 minutes total. (You may not have to use all the liquid.)
Remove from heat. Stir in spinach, parmesan, and butter; season with salt and pepper. Serve immediately (risotto will thicken as it cools), and sprinkle with additional parmesan, if desired.