Heat the olive oil in a 2-quart saucepan over medium heat. Add the Italian sausage and cook, breaking it apart with a spoon, until browned and cooked through, 5 to 7 minutes.
Add the onion and cook until softened, about 2 minutes. Stir in the garlic, thyme, salt, and pepper and cook until fragrant, about 1 minute.
Pour in the chicken broth and add the chopped tomato. Bring to a boil, then reduce the heat and simmer for 10 minutes.
Stir in the tortellini and simmer until tender, 3 to 5 minutes.
Add the spinach and heavy cream. Simmer for 2 minutes, stirring occasionally, until the spinach wilts.
Ladle the soup into a bowl and top with grated Parmesan, if using.
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Notes
Don't overcook the tortellini. Add them in the last few minutes and simmer just until tender, 3 to 5 minutes for refrigerated. Left too long in the broth, they turn soft and lose their shape.Add the cream off the boil. Stir the heavy cream in after you lower the heat. A hard boil can cause it to separate, leaving flecks in the broth instead of a smooth, even texture.Cook the tortellini separately if you want leftovers. The pasta soaks up the broth as it sits, so if you know you'll have some for later, cook the tortellini on their own and add them to each bowl. Store the broth and tortellini separately.Match the pot to the batch. A 2-quart saucepan is right for a single serving. If you double the recipe, move up to a 3.5 to 4-quart pot so there's room to simmer.If doubling the recipe, double every ingredient and use a 3.5 to 4-quart pot. The cooking times stay about the same.