Heat a pot over medium-high heat, and add the olive oil, sausage, garlic, onion, carrots, and celery. Stir and cook for 4-5 minutes.
Stir the butter into the mixture until it melts; then, add in the diced tomatoes, thyme, tomato paste, and broth. Remove the pot from the heat, add the cornstarch and use a spoon to stir it into the broth. Place back onto the heat and allow to simmer for 15-20 minutes.
Stir the tortellini into the soup and cook for 5 minutes.
Pour in the heavy cream and chopped spinach, stir, and let simmer for 5 minutes or until the spinach is wilted. Season with salt and pepper, if desired.