Step 1: Prepare the Chicken and Vegetables - Cut the chicken thighs into bite sized pieces so they cook evenly and are easy to spoon over noodles, rice, or potatoes. Slice the onion, halve the mushrooms, and mince the garlic. I like getting all of this done before I begin adding anything to the slow cooker because it keeps the counter organized and makes the rest of the recipe feel almost effortless.
Step 2: Fill the Slow Cooker - Add the chicken pieces to the slow cooker, followed by the sliced onion and halved mushrooms. Pour in the chicken broth, Worcestershire sauce, and Dijon mustard. Sprinkle the paprika, salt, and black pepper over everything, then add the minced garlic and butter. Give the mixture a thorough but gentle stir so the mustard and seasonings are distributed throughout the broth.
Step 3: Cook Until the Chicken Is Tender - Cover the slow cooker and cook on Low for 6 hours or on High for 3 hours. Both settings work well, so choose the one that fits your day. Low is helpful when you are starting the meal in the morning, while High works when you remember dinner sometime after lunch. You can stir the mixture gently around the halfway point, but it is not essential.
Step 4: Add the Heavy Cream - About 1 hour before the cooking time is complete, pour the heavy cream into the slow cooker. Stir slowly until the cream blends with the broth and turns the cooking liquid pale, smooth, and rich. Adding it near the end gives you a creamier sauce and keeps the dairy from cooking longer than necessary.
Step 5: Mix the Cornstarch Slurry - Place the cornstarch in a small bowl and add the cold water. Whisk until the mixture is completely smooth and no dry pockets remain. This is called a slurry, and it is the little step that turns the thin cooking liquid into a sauce. Do not add dry cornstarch directly to the slow cooker because it can form stubborn lumps.
Step 6: Thicken the Stroganoff Sauce - Pour the cornstarch slurry into the slow cooker and stir well. Cover the slow cooker again and continue cooking for the remaining hour. As the sauce heats, it will gradually thicken and begin clinging to the chicken and mushrooms. Slow cookers vary, so the sauce may look slightly thinner while it is very hot and become thicker as it rests.
Step 7: Taste and Finish the Dish - Once the chicken is tender and the sauce has thickened, taste a spoonful and adjust the seasoning. You may want another small pinch of salt, a little black pepper, or an extra spoonful of Dijon if your family enjoys more tang. Sprinkle chopped fresh parsley over the top just before serving. I usually bring the slow cooker right to the table because the creamy sauce and bright parsley make it look especially inviting.