In a small saucepan over medium heat, bring diced tomatoes (in juice) and water to a simmer. Lower heat and keep mixture warm.
In a separate large saucepan over medium heat, add olive oil. When oil is hot, add in sausage and onion. Using a wooden spoon or spatula, break up the sausage as it cooks. Cook for about 5 minutes until sausage is cooked through and onions are soft.
Add arborio rice to the saucepan and cook for about 2 minutes, stirring a bit, until it’s translucent around the edges. Add in white wine and stir a bit (scraping up any bits from the bottom of the saucepan) until the wine is totally absorbed, 2 minutes.
Add about 2 cups of the warm tomato mixture and stir occasionally until it has absorbed into rice completely, 3-4 minutes. Continue adding the mixture 1 cup at a time, stirring after each addition, and waiting to add more until liquid is all absorbed. Stir occasionally. When rice is creamy and al dente, you can stop (you may not need to add all the liquid).
Remove saucepan from heat and mix in salt and pepper, chopped spinach, parmesan cheese, and butter.
Serve risotto in bowls and top with additional parmesan cheese.