Cook the Italian sausage in a large dutch oven or stock pot over medium-high heat for 5-7 minutes or until no longer pink, breaking up the sausage with a wooden spoon as it cooks.
Meanwhile: prepare the celery, carrots, onion, and garlic.
Reserve 1 tablespoon of the sausage grease. Transfer the cooked sausage to a paper towel-lined plate using a slotted spoon and drain the excess grease from the Dutch oven.
Add the celery, carrots, and onion to the pan and cook for 4-6 minutes or until the vegetables soften.
Add the garlic and tomato paste and cook, stirring continuously, for 1-2 minutes or until fragrant.
Add the cooked sausage, stock, canned tomatoes, Italian seasoning, salt, pepper, and chili flakes to the pan and bring to a gentle boil. Reduce the heat to medium and simmer for 10 minutes to allow the flavors to develop.
Stir in the gnocchi and cook for 4-5 minutes or until the gnocchi floats to the surface and is fork-tender.
Remove from the heat and stir in the baby spinach. Allow the soup to sit for 2-3 minutes to let the spinach wilt.
Serve hot with freshly grated parmesan if desired.
Notes
Store leftovers in the refrigerator for up to three days. Reheat the soup in the microwave or a saucepan over medium heat. The soup can be frozen in an airtight container or freezer bag for up to three months. Allow the soup to cool to room temperature before freezing. Thaw the soup in the refrigerator before reheating. The gnocchi will absorb some of the liquid the longer the soup sits. I like to add a splash more chicken broth or water to the soup when reheating it to replace the liquid that has been absorbed.I used mild Italian sausage in the soup; however, you can use hot Italian sausage instead if you prefer your soup with more of a kick. Turkey sausage can also be used in the soup; however, you will need to add a tablespoon of oil to the pan before sauteeing the vegetables, as it is a lot leaner than traditional pork sausage. You can use full-size spinach instead of baby spinach, just roughly chop it before adding it to the soup.Roughly chopped kale can be used instead of spinach in this soup. If using kale, I recommend adding it at the same time as the gnocchi, as it takes longer to break down than spinach does. If you do not have Italian seasoning, you can add ½ a teaspoon each of dried oregano, basil, and thyme.
Gnocchi can be found in the dried pasta aisle at the grocery store.