Place halved potatoes in a pot of cold salted water. Bring to a boil and simmer for 10 minutes or until parboiled. Drain and set aside.
Season chicken breasts on both sides with salt and black pepper.
Heat olive oil in a large heavy-bottomed skillet over medium-high heat. Sear chicken for 5-6 minutes per side until golden brown and a minimum internal temperature of 74°C (165°F) is reached. Remove chicken and tent with foil.
In the same skillet, sauté the diced onion for 3 minutes until translucent. Add minced garlic and cook for 60 seconds until fragrant.
Deglaze the pan with chicken stock, scraping the fond from the bottom of the skillet.
Reduce heat to medium-low. Whisk in the Dijon mustard, whole grain mustard, heavy cream, and dried thyme.
Add the parboiled potatoes to the skillet. Simmer for 5 minutes until the sauce reduces and thickens to a coat-the-back-of-a-spoon consistency.
Return the chicken and any accumulated juices to the pan. Spoon the sauce over the chicken and heat through for 2 minutes.
Garnish with fresh parsley and serve immediately.