Step 1: Season the Chicken - Pat the chicken thighs dry with paper towels, then season both sides with the kosher salt, black pepper, and garlic powder. Drying the surface helps the seasonings stick and allows the chicken to brown instead of steaming in the skillet. I usually season everything on a large plate so I can move straight to the stove without dirtying another bowl.
Step 2: Sear the Chicken - Heat the oil in a large skillet over medium high heat. Once the oil is hot and shimmering, add the chicken in a single layer and sear it for about 2 minutes on each side. You are not trying to cook it through at this stage. You only want a light golden surface, which adds a deeper roasted flavor and helps the thighs hold their shape while they cook.
Step 3: Fill the Slow Cooker - Transfer the seared chicken to a 6 quart slow cooker, arranging the pieces in an even layer. Scatter the sliced onion over the chicken. Do not worry if some of the slices fall between the thighs because they will soften into the gravy as everything cooks. If your slow cooker runs hot, placing the onions on top rather than underneath also keeps them from darkening too quickly.
Step 4: Mix the Savory Gravy - In a medium bowl, whisk together the cream of mushroom soup, brown gravy mix, beef broth, Worcestershire sauce, and onion powder. Keep whisking until the gravy mix is dissolved and there are no dry pockets hiding around the edge of the bowl. The mixture will look thinner than finished gravy, which is exactly what you want because it will concentrate during the long cooking time.
Step 5: Add the Gravy and Mushrooms - Pour the gravy mixture evenly over the chicken and onions. Use a spoon to make sure each piece of chicken gets coated, then scatter the sliced baby bella mushrooms across the top. The mushrooms may look like a lot at first, but they shrink as they cook and release their savory juices directly into the sauce.
Step 6: 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 chicken is ready when it reaches 165°F in the center and feels tender when pressed with a fork. Slow cookers vary, so begin checking near the early end of the cooking range, especially if your chicken thighs are small.
Step 7: Remove and Rest the Chicken - Carefully move the cooked chicken to a plate and cover it loosely with foil. The thighs will be very tender, so a wide spatula or large serving spoon is helpful here. Moving the chicken out temporarily gives you room to whisk and thicken the gravy without breaking the meat into small pieces.
Step 8: Thicken the Gravy - In a small bowl, whisk the cornstarch and cold water together until completely smooth. This mixture is called a slurry, and using cold water is important because hot water can cause the cornstarch to form stubborn lumps. Stir the slurry into the liquid in the slow cooker, scraping gently around the sides and bottom so it is distributed evenly.
Step 9: Finish With Cream - Pour the heavy cream into the slow cooker and stir until the gravy looks smooth and evenly colored. Cover and cook on HIGH for another 10 to 15 minutes, or until the sauce has thickened enough to coat the back of a spoon. It will continue to thicken slightly as it sits, so give it a minute before deciding it needs more cornstarch.
Step 10: Return the Chicken and Serve - Place the chicken back into the slow cooker and spoon the creamy mushroom gravy over the top. Let it sit in the warm gravy for a few minutes so every piece is heated through and well coated. Sprinkle with chopped parsley, if you are using it, and bring the slow cooker right to the table or serve the chicken over mashed potatoes, egg noodles, or rice.