If using the sausage, heat the olive oil in a large pot or Dutch oven over medium-high (if not using sausage, heat the oil on medium, then proceed to Step 2.) Add the sausage and cook, breaking it up with a spoon as you go, until it is browned, 5 to 9 minutes. Push the sausage to one side of the pot and spoon out any excess grease, leaving about 2 tablespoons behind.
If necessary, adjust heat to medium, then add onion, garlic, paprika, fennel, 1½ teaspoons each salt and black pepper, and a pinch of crushed red pepper, and cook, stirring, for 2 minutes, until the onion is just softened and everything is fragrant. Add the tomato paste and cook, stirring, until it darkens, 2 to 3 minutes. Adjust heat to high, stir in tomato purée and stock, cover the pot and bring to a gentle boil, then add the tortellini and cook until tender, 2 to 3 minutes (1 minute longer if frozen).
Add the heavy cream, kale and basil, and cook, stirring, until warmed through and the kale and basil are just wilted, 2 minutes more. Remove from heat and add the lemon juice. Stir to incorporate, then taste and season with salt and pepper as needed.
Serve warm topped with plenty of grated Parmesan. Soup can be refrigerated for up to 5 days or frozen for up to 3 months.
