Step 1: Brown the Chicken & Mushrooms (Optional but Highly Recommended)
Pat the chicken thighs completely dry and season with salt and pepper. In a large skillet, heat 1 tablespoon of oil or butter over medium-high heat. Brown the chicken thighs for 3-4 minutes per side, until golden. Transfer to the slow cooker. In the same skillet, add another tablespoon of fat if needed. Add the sliced mushrooms and cook for 5-7 minutes until they have released their liquid and are deeply browned. In the last minute, add the diced onion and minced garlic, cooking until fragrant. This step builds immense flavor.

Step 2: Layer the Aromatics in the Slow Cooker
If you skipped browning, simply place the raw chicken in the slow cooker and scatter the raw mushrooms, onion, and garlic on top. Pour the chicken broth and white wine (if using) into the skillet used for browning (or a bowl if you didn’t brown) to deglaze, scraping up any browned bits. Stir in the tomato paste, Worcestershire sauce, Dijon mustard, thyme, onion powder, salt, and pepper until combined. Pour this mixture over the chicken and mushrooms in the slow cooker.

Step 3: The Long, Slow Simmer
Cover and cook on LOW for 6-7 hours or on HIGH for 3-4 hours. The chicken is done when it is fall-apart tender.

Step 4: Shred the Chicken and Thicken the Sauce
Using two forks, carefully shred the chicken directly in the slow cooker into bite-sized pieces. In a small bowl, whisk the flour (or cornstarch) with the ¼ cup of cold water until completely smooth to create a slurry. Stir this slurry into the slow cooker. Cover and cook on HIGH for 20-30 minutes, until the sauce has thickened noticeably.

Step 5: Create the Creamy Sauce
Reduce the slow cooker to the WARM setting (or turn it off). In a medium bowl, whisk together the room-temperature heavy cream and sour cream (or yogurt) until smooth. Gradually whisk about 1 cup of the hot liquid from the slow cooker into the cream mixture to temper it (this prevents curdling). Then, pour this tempered cream mixture back into the slow cooker, stirring constantly until fully incorporated.

Step 6: Enrich with Cheese and Final Seasoning
Stir in the grated Parmesan cheese (if using) until melted. Taste the sauce critically. Adjust seasoning with additional salt and a pinch of white pepper for a clean, sharp finish. The sauce should be creamy, savory, and well-balanced.

Step 7: Rest and Garnish
Let the dish sit, covered, for 10-15 minutes to allow the sauce to thicken slightly further and the flavors to fully marry. The sauce will continue to thicken as it stands.

Step 8: Serve and Enjoy
Serve the creamy mushroom chicken over a bed of buttery egg noodles, fluffy mashed potatoes, steamed rice, or with crusty bread to soak up the exquisite sauce. Garnish generously with fresh chopped parsley and a sprig of thyme