Heat 1 tablespoon extra-virgin olive oil in Dutch oven over medium heat until shimmering.
Add 1 pound hot or sweet Italian sausage, casings removed, and 1 finely chopped onion and cook, breaking up meat with wooden spoon, until sausage is starting to brown and onion is softened, 5 to 7 minutes.
Add 3 minced garlic cloves and ¼ teaspoon red pepper flakes and cook until fragrant, about 30 seconds.
Stir in 4 cups chicken broth and 1 can crushed tomatoes and bring to simmer, scraping up any browned bits.
Cover, reduce heat to low, and simmer gently for 10 minutes, stirring occasionally.
Remove lid, stir in 1 package fresh or frozen cheese tortellini, and simmer, stirring frequently, until tender.
Off heat, stir in 5 ounces baby kale and ½ cup heavy cream, if using.
Season with salt and pepper to taste and serve with grated Parmesan cheese.
