tomato and sausage risotto

INGREDIENTS

• 1 can (28 ounces) diced tomatoes in juice
• 1 tablespoon olive oil
• 3/4 pound sweet or hot Italian sausage, casings removed
• 1 small onion, finely chopped
• Coarse salt and ground pepper
• 1 cup Arborio rice
• 1/2 cup dry white wine
• 1 bunch flat-leaf spinach (10 to 14 ounces), washed well, tough stems removed, chopped (about 7 cups)
• 1/2 cup grated Parmesan cheese, plus more for serving (optional)
• 2 tablespoons butter