Combine Ingredients in Slow Cooker
In a 6 to 8-quart slow cooker, add the barley, sliced mushrooms, chicken thighs, chicken broth, white wine, and lemon juice. Incorporate the smashed garlic, thyme leaves, salt, onion powder, red-pepper flakes, and a few grinds of black pepper. Stir to combine, ensuring the ingredients are well-distributed. Cover and cook on high for 3 hours.
Stir and Add Frozen Peas
Upon completion of the cooking time, you may notice an abundance of liquid. Do not be concerned, as the mixture will thicken as it cools and is stirred. Once the liquid reduces, add the frozen peas, stirring them through and allowing them to heat for approximately 2 minutes.
Finish the Dish and Serve
In a small bowl, blend the crème fraîche (or sour cream) with Dijon mustard. Gently fold this creamy mixture into the cooked barley and chicken. As the chicken thighs should be tender and nearly falling apart, break them into large chunks using a spatula. Adjust the mustard to taste, adding more for a sharper flavor if desired. Incorporate the chopped tarragon or dill, stir well, and serve. Pass grated Parmesan around for topping.