Easy Parmesan Sausage Soup (Cozy One-Pot Dinner)

Ingredients

• 1 tablespoon olive oil
• 1 lb (450 g) mild or hot Italian sausage, casings removed if needed
• 2 tablespoons unsalted butter
• 1 medium yellow onion, finely diced
• 2 medium carrots, finely diced
• 2 celery ribs, finely diced
• 4 cloves garlic, minced
• 1 teaspoon Italian seasoning
• 1/4 teaspoon red pepper flakes, optional
• 1/2 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 2 tablespoons all-purpose flour
• 6 cups (1.4 L) low-sodium chicken broth
• 1 tablespoon tomato paste
• 1 can (14.5 oz/410 g) diced tomatoes, with juices
• 1 1/4 cups (150 g) ditalini or other small pasta
• 1 cup (240 ml) heavy cream
• 1 cup (90 g) freshly grated Parmesan cheese, plus more for serving
• 3 packed cups (90 g) baby spinach
• 2 tablespoons chopped fresh parsley, optional