Step 1: Prepare the Slow Cooker - Lightly coat the inside of a 6 quart slow cooker with nonstick cooking spray. You do not need a heavy layer. A quick, even coating is enough to make serving and cleanup easier, especially around the edges where the cheese can settle and brown.
Step 2: Add and Season the Chicken - Place the bite size chicken pieces in an even layer across the bottom of the slow cooker. Sprinkle the salt, black pepper, and garlic powder over the chicken, then scatter the diced onion on top. Keeping the chicken near the bottom puts it closest to the heat and allows it to cook in the broth and creamy sauce.
Step 3: Build the Creamy Pot Pie Base - Add the 2 tablespoons of butter and pour in the chicken broth. Spoon in the cream of chicken soup, then sprinkle in the chicken gravy mix and poultry seasoning. Stir gently around the chicken until the soup and broth begin to combine. The mixture may not look completely smooth yet, and that is fine. Heat will bring everything together as it cooks.
Step 4: Add the Cream Cheese and Cook - Place the cream cheese cubes across the top so they are spread out rather than piled in one spot. Cover the slow cooker and cook on low for 3 to 4 hours, or on high for 2½ to 3 hours. The chicken should be fully cooked, tender, and at least 165°F in the center. Try not to lift the lid often, since every peek releases heat and slows the cooking.
Step 5: Stir the Chicken and Sauce - Once the chicken is cooked through, remove the lid and stir the mixture well. The cream cheese should soften into the soup and broth, creating a thick, creamy sauce. If you see a few small white streaks, keep stirring for another minute. They will melt into the filling as the casserole continues to cook.
Step 6: Add the Hash Browns and Vegetables - Stir in the thawed shredded hash browns, frozen peas and carrots, and frozen corn. Take a moment to fold everything from the bottom upward so the potatoes and vegetables are evenly coated in sauce. Cover and cook on low for 1½ to 2 hours, stirring once halfway through, until the hash browns are tender.
Step 7: Melt in the Cheddar - Add 2 cups of shredded sharp cheddar and stir until the cheese disappears into the hot filling. The casserole should look thick, creamy, and scoopable, with tender strands of potato throughout. Taste a small spoonful once it is cool enough and add a pinch of salt or pepper only if your broth and soup were mild.
Step 8: Make the Buttery Cracker Topping - In a small bowl, stir the crushed Ritz crackers with the 3 tablespoons of melted butter. Mix until the crumbs look evenly moistened. Sprinkle the remaining one half cup of cheddar over the casserole, then scatter the buttery crumbs across the cheese. An even layer gives each serving a little bit of that soft, crisp topping.
Step 9: Finish and Rest the Casserole - Cover and cook for another 10 to 15 minutes, just until the final layer of cheese melts and the casserole is hot throughout. Sprinkle with parsley if you are using it, then let the casserole rest uncovered for 5 to 10 minutes. That brief rest helps the sauce settle and thicken, so it stays on the spoon instead of running across the plate.