Step 1: Season the Chicken - Add the cubed chicken to the slow cooker, then sprinkle in the salt, black pepper, garlic powder, onion powder, and poultry seasoning. Stir so the seasonings coat the chicken as evenly as possible. Starting with seasoned chicken matters because it gives those little pieces flavor all the way through, not just on the outside once they are sitting in the broth.
Step 2: Add the Butter and Fresh Vegetables - Add the butter, diced onion, celery, minced garlic, and diced potatoes to the slow cooker. Keep the potato pieces fairly small and similar in size so they finish cooking at the same time as the chicken. I usually aim for pieces small enough to fit comfortably on a spoon, because nobody wants to chase a giant potato chunk around a bowl of hot soup.
Step 3: Build the Creamy Soup Base - Spoon in the cream of chicken soup and cream of celery soup. Pour in the chicken broth, then add the bouillon cube, frozen mixed vegetables, frozen corn, and bay leaf. Stir everything well, taking a moment to break up the condensed soups so there are no large pockets left around the chicken or potatoes. The mixture may not look completely smooth yet, but it will come together as it heats.
Step 4: Slow Cook Until Tender - Cover the slow cooker and cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours. The soup is ready for the finishing ingredients when the chicken is cooked through and the potatoes are tender enough to cut easily with the side of a spoon. Slow cookers can vary, so check near the early end of the time range, especially if yours tends to run hot.
Step 5: Remove the Bay Leaf and Add Cream - Find and discard the bay leaf, then stir in the heavy cream. Adding the cream near the end keeps the broth smooth and gives it a fresh, rich finish. At this point, the soup should already smell like the center of a chicken pot pie, and this is usually when my family starts wandering into the kitchen asking how much longer dinner will take.
Step 6: Mix and Add the Cornstarch Slurry - In a small bowl, whisk the cornstarch with the cold water until the mixture is completely smooth. Pour it into the slow cooker while stirring the soup. Always mix cornstarch with cold water first, since adding dry cornstarch directly to hot soup can leave little lumps that are hard to remove.
Step 7: Thicken the Soup - Cover the slow cooker again and cook for another 20 to 30 minutes, or until the broth has thickened to a creamy pot pie consistency. Give it a stir once or twice if you are nearby. The soup will continue to thicken a little as it rests, so do not worry if it still looks slightly looser than traditional pie filling.
Step 8: Bake the Puff Pastry - While the soup thickens, cut the thawed puff pastry into six squares and bake them according to the package directions. Space the squares apart on the baking sheet so hot air can move around them and help the layers rise. Bake until they are puffed, deeply golden, and crisp. If you are using biscuits instead, bake them according to the package directions.
Step 9: Finish and Serve - Stir the chopped parsley into the soup, then taste and adjust the salt and pepper if needed. Ladle the hot soup into bowls and place one fresh baked puff pastry square on top of each serving. I like to bring any extra pastry to the table, because the crisp layers are made for breaking apart and dipping into that creamy broth.