In a small saucepan, combine tomatoes (with their juice) and 3 cups water. Bring just to a simmer; keep warm over low heat.
In a medium saucepan, heat oil over medium. Add sausage and onion; season with salt and pepper. Cook, breaking up sausage with a spoon, until sausage is opaque and onion has softened, 3 to 5 minutes.
Add rice; cook, stirring until well coated, 1 to 2 minutes. Add wine; cook, stirring until absorbed, about 1 minute.
Add about 2 cups 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 one cup 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 pan 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.