Step 1: Prepare the Slow Cooker and Vegetables

• Lightly grease the insert of a 6-quart or larger slow cooker with cooking spray or a thin layer of olive oil. This will prevent the casserole from sticking and make cleanup much easier.
• Add the diced yellow onion, sliced carrots, diced celery, and sliced cremini mushrooms to the bottom of the slow cooker. Spread them out to create an even layer. This bed of vegetables will release their flavors as they cook, infusing the entire dish.

Step 2: Add the Chicken and Garlic

• Place the boneless, skinless chicken thighs on top of the vegetable layer. You can use whole thighs; they will become incredibly tender and easy to shred after cooking.
• Sprinkle the minced garlic over the chicken.

Step 3: Whisk Together the Sauce

• In a medium-sized mixing bowl, combine the gluten-free condensed cream of chicken soup, chicken broth, sour cream (or Greek yogurt), dried thyme, dried rosemary, onion powder, paprika, salt, and black pepper.
• Whisk vigorously until the sauce is completely smooth and all the ingredients are well incorporated. There should be no lumps from the condensed soup.

Step 4: Combine Everything in the Slow Cooker

• Pour the prepared creamy sauce evenly over the chicken and vegetables in the slow cooker. Use a spatula to gently nudge the chicken and vegetables to ensure the sauce seeps down and covers everything. This is crucial for ensuring every bite is flavorful and moist.

Step 5: Cook Low and Slow

• Place the lid on the slow cooker.
• Cook on the LOW setting for 6 to 8 hours, or on the HIGH setting for 3 to 4 hours. The low and slow method is highly recommended as it results in the most tender, fall-apart chicken. The chicken is done when it is cooked through and easily shreds with a fork.

Step 6: Shred the Chicken

• Once the cooking time is complete, carefully remove the chicken thighs from the slow cooker and place them on a cutting board or in a large bowl.
• Using two forks, shred the chicken into bite-sized pieces. It should be extremely tender and shred with very little effort.
• Return the shredded chicken to the slow cooker.

Step 7: Add the Final Ingredients

• Add the 1 cup of frozen green peas to the slow cooker. There is no need to thaw them first; the residual heat of the casserole will cook them perfectly.
• Stir everything together until the shredded chicken and peas are evenly distributed throughout the creamy sauce and vegetables.

Step 8: Add the Cheese Topping and Melt

• Sprinkle the 1 ½ cups of shredded sharp cheddar cheese evenly over the top of the casserole.
• Place the lid back on the slow cooker and cook on the HIGH setting for another 10-15 minutes, or just until the cheese is completely melted, bubbly, and delicious.
• Turn off the slow cooker. Let the casserole rest for 5-10 minutes before serving. This allows the sauce to thicken up slightly. Garnish with fresh parsley if desired.