In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion, carrot, and celery, and sauté until the vegetables are soft, about 5 minutes.
Add the minced garlic and cook for an additional minute until fragrant.
Add the Italian sausage to the pot. Break it up with a wooden spoon and cook until it is browned and cooked through, about 5-7 minutes.
Pour in the chicken broth and add the canned diced tomatoes (with their juice) to the pot. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Add the dried Italian seasoning, salt, and pepper to taste.
Stir in the cheese tortellini and cook according to package instructions, usually about 3-5 minutes.
Add the fresh spinach to the pot and stir until it wilts, about 1-2 minutes.
Taste and adjust seasoning if necessary before serving.