Step 1: Season the Chicken - Place the chicken breasts in the bottom of your slow cooker. Sprinkle them evenly with the salt, black pepper, garlic powder, poultry seasoning, and onion powder, then add the minced garlic. I like getting the seasoning directly onto the chicken instead of simply stirring everything into the sauce because the meat picks up more flavor as it cooks. You do not need to sear the chicken first. That would only add another pan to wash, and for this particular dinner, the slow cooker does a fine job without it.
Step 2: Add the Vegetables - Pour the frozen onion, celery, and carrot mixture over the seasoned chicken, followed by the frozen mixed vegetables. There is no need to thaw anything first. That is part of what makes this meal so useful on busy days. Spread the vegetables around so they form a fairly even layer. As they cook, they will soften and release some moisture into the sauce while still giving each bowl plenty of texture and color.
Step 3: Whisk the Creamy Sauce - In a medium bowl, whisk together the cream of chicken soup, cream of celery soup, chicken broth, and heavy cream until everything is smooth and combined. The mixture may look fairly loose right now, and that is exactly what you want. It will thicken as it cooks and as the shredded chicken gets stirred back into the filling. Pour the sauce evenly over the vegetables and chicken, making sure it reaches most of the slow cooker.
Step 4: Add the Butter and Slow Cook - Place the butter on top of the filling, then cover the slow cooker with the lid. Cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours. I usually choose LOW when I can because it gives the chicken plenty of time to become tender, but either setting works. The chicken is ready when it is fully cooked and pulls apart easily with a fork. Try not to lift the lid repeatedly while it cooks because every peek releases heat and adds time.
Step 5: Shred the Tender Chicken - Carefully remove the cooked chicken from the slow cooker and transfer it to a plate or cutting board. Use two forks to shred it into bite sized pieces. If the chicken is properly tender, this should take almost no effort. Return all that shredded chicken to the slow cooker and gently stir it into the vegetables and creamy sauce. At this point, the filling should look thick, hearty, and very much like the inside of a chicken pot pie.
Step 6: Cut and Arrange the Biscuits - Open the refrigerated biscuits and cut each biscuit into eight roughly equal pieces. Scatter those pieces across the top of the filling in a single layer. This part matters. Do not stir the raw biscuit dough down into the sauce. Keeping the dough on top allows the hot steam inside the slow cooker to surround the biscuit pieces and help them rise into fluffy dumplings instead of turning heavy and gummy in the filling.
Step 7: Steam the Biscuit Dumplings - Cover the slow cooker again and turn the heat to HIGH. Cook the biscuit pieces for 45 to 60 minutes, or until they are puffed and cooked through in the center. Resist lifting the lid while they cook. I know it is tempting when the whole kitchen smells like pot pie, but you want to keep that steam trapped inside. If you know your slow cooker tends to run hot, begin checking around the 40 minute mark so the biscuits do not overcook around the edges.
Step 8: Finish and Serve - Once the biscuits are fluffy and no raw dough remains in the center, sprinkle chopped fresh parsley over the top. Let the filling sit for a few minutes before spooning it into bowls because it will be very hot straight from the slow cooker. I like to finish each serving with another crack of black pepper. Make sure every bowl gets a generous scoop of chicken and vegetables along with several biscuit dumplings, because those sauce soaked biscuit bottoms are too good to miss.