Season 2 pounds boneless, skinless chicken thighs with 1 teaspoon salt and ½ teaspoon black pepper, then place in the bottom of the slow cooker.
Scatter the sliced 1 small onion, 3 carrots, 2 stalks celery, 8 ounces mushrooms, and 3 cloves garlic over the chicken.
In a small bowl, whisk together 1 cup merlot, ½ cup chicken broth, 2 tablespoons tomato paste, W1 tablespoon Worcestershire sauce, 1 teaspoon Italian seasoning, and½ teaspoon dried thyme. Pour over the chicken and veggies.
Cover and cook on Low 6–7 hours or High 3–4 hours, until the chicken is fork-tender and infused with the sauce.
Mix 1 tablespoon cornstarch and 1 tablespoon cold water; stir into the liquid in the slow cooker. Cover and cook on High for 10–15 minutes until slightly thickened. Stir in 2 tablespoons butter if desired for extra richness.
Spoon the chicken and sauce over mashed potatoes or noodles and garnish with Fresh parsley