Season chicken breasts with salt and black pepper, then arrange them snugly in your slow cooker.
In a small bowl, whisk together Dijon mustard, coarse mustard, minced garlic, thyme, chicken stock, and honey. Pour this mixture over the chicken.
Set your slow cooker to LOW and cook for 4 hours, or until the chicken is fully cooked and tender.
Gently remove the chicken breasts from the slow cooker and set them aside on a plate.
Stir in the heavy cream, lemon juice, and Parmesan cheese to the sauce in the slow cooker.
In a small bowl, mix cornstarch and water to create a slurry. Slowly stir it into the sauce to thicken it. Turn the slow cooker to HIGH and let the sauce cook for 10 minutes until it thickens slightly.
Return the chicken breasts to the slow cooker, coating them in the creamy mustard sauce before serving.
