1. Season the chicken breasts with salt and pepper generously.
2. In a large skillet over medium heat, heat the olive oil and brown the chicken breasts on both sides, about 2-3 minutes per side. Transfer to the slow cooker.
3. In the same skillet, add the minced garlic and sauté briefly until fragrant, about 1 minute. Pour in the chicken broth and bring to a simmer while scraping any browned bits from the pan.
4. Stir in the whole grain mustard and Dijon mustard, and then pour the mixture over the chicken in the slow cooker.
5. Cover and cook on low for 4-5 hours, or until the chicken is tender and cooked through.
6. When the cooking time is almost complete, whisk together the heavy cream and sour cream in a small bowl. Pour this mixture into the slow cooker, stirring gently to combine.
7. Add the chopped thyme and let cook for an additional 20-30 minutes until the sauce is creamy and heated through.
8. Serve the chicken with a sprinkle of fresh parsley as a garnish.
