Heat olive oil in a large cast iron skillet over medium high heat. Add Italian sausage and cook until browned, about 3-5 minutes, making sure to crumble the sausage as it cooks; drain excess fat.
Place Italian sausage into a 6-qt slow cooker. Stir in chicken stock, diced tomatoes, tomato sauce, onion, garlic, Italian seasoning and Parmesan rind, if using; season with salt and pepper, to taste.
Cover and cook on low heat for 7-8 hours or high heat for 3-4 hours.
Uncover and stir in tortellini, kale and heavy cream; cover and cook on high heat for an additional 20-30 minutes, or until tortellini is tender. Stir in basil; season with salt and pepper, to taste.
Serve immediately, topped with Parmesan.
Notes
Brown the sausage first. Brown and crumble the sausage in a large cast iron skillet prior to adding to the slow cooker, draining the excess fat to avoid an overly oily soup.
Use good-quality stock. The better quality your stock, the better your soup. Many brands love to pack in the salt so always opt for unsalted or a lower-sodium variety. Homemade stock is even better.
Save leftover Parmesan rinds. Get more out of those expensive Parmesan cheeses by using up the leftover rinds! Adding a Parmesan rind slightly thickens the broth and adds a level of nutty richness to the soup.
Tortellini cooks quickly. Always add the refrigerated tortellini towards the end to keep it al dente and tender. The longer it sits in the broth, the more it will soak up all the liquid and turn mushy over time.
Sneak in those leafy greens. This is a great recipe to sneak in leafy greens for those picky eaters. Spinach, kale, collard greens, swiss chard, or arugula are all great options, adding towards the end of cook time.
Add the cream slowly. Always add the heavy cream at the very end (prolonged cooking can cause the heavy cream to separate and become grainy), pouring very slowly to prevent curdling.
Make it lighter. Wanting a lighter soup? Substitute half-and-half or skip the heavy cream altogether.
Store tortellini separately for leftovers. When planning for leftovers, store the cooked tortellini separately from the soup to prevent it from soaking up all the broth.
Make it on the stovetop. Need a quicker stovetop version? Try this recipe instead.