Step 1: Add the Vegetables - Place the finely diced onion, shredded carrots, minced garlic, and chopped broccoli in the slow cooker. I like to chop the broccoli into fairly small pieces because it makes the finished soup much easier to eat. You want a little broccoli on your spoon alongside the tortellini, not one giant floret that takes over the whole bite. There is no need to cook the vegetables first. The slow cooker will soften everything for you.
Step 2: Build the Broth - Pour the chicken broth over the vegetables, then add the condensed cheddar cheese soup, garlic powder, onion powder, kosher salt, black pepper, and paprika. Give everything a good stir so the condensed soup begins mixing into the broth and the seasonings are distributed throughout the slow cooker. Do not worry if the mixture does not look perfectly smooth yet. As it heats, the cheddar soup and butter will melt into the broth.
Step 3: Add the Butter and Slow Cook - Place the butter on top of the mixture, cover the slow cooker, and cook on LOW for 4 to 5 hours or HIGH for 2 to 3 hours. The vegetables should be tender before you move on. I usually check a piece of carrot and broccoli with a fork. If they give easily, you are ready for the creamy part. This is the stretch where the slow cooker earns its spot on the counter because you can walk away and let dinner quietly take care of itself.
Step 4: Stir In the Cream - About 30 minutes before you plan to serve the soup, stir in the heavy cream. Give it a gentle stir so the cream blends with the vegetable broth. At this point the soup starts looking noticeably richer and creamier, but keep in mind that it still has cheddar and cornstarch coming. If it looks slightly thinner than you expected right now, that is completely normal.
Step 5: Melt In the Cheddar - Add the freshly shredded sharp cheddar a handful at a time, stirring gently after each addition. Giving each handful a chance to melt before adding more helps create a smoother soup. Keep the heat gentle and avoid letting the soup boil once the cheese is in. High heat can make dairy based cheese sauces turn grainy, and after waiting all afternoon for dinner, nobody wants the cheddar to get cranky at the finish line.
Step 6: Add the Tortellini - Stir the frozen cheese tortellini gently into the soup. Make sure most of the pasta is tucked down into the hot broth so it can cook evenly. Tortellini does not need hours in the slow cooker, which is why it joins the party at the end. Adding it early would give the pasta too much time to soften and could leave you with broken or mushy tortellini instead of those nice tender cheese filled pockets.
Step 7: Thicken and Finish the Soup - In a small bowl, whisk the cornstarch and cold water together until completely smooth. Pour that mixture into the slow cooker and stir gently. Cover and cook on HIGH for another 20 to 30 minutes, checking the tortellini toward the beginning of that range because different brands cook at different speeds. Once the tortellini is tender and the broth has thickened, give the soup one final gentle stir and taste for seasoning. Ladle it into bowls and finish each serving with a little extra shredded cheddar.