Roast the Potatoes:
Preheat oven to 400°F (200°C). Toss halved baby potatoes with 1 tablespoon olive oil, salt, and pepper. Spread on a baking sheet and roast for 20–25 minutes, flipping once, until golden and tender.
Sear the Chicken:
While potatoes roast, season chicken with salt and pepper. Heat 1 tablespoon olive oil in a large skillet over medium heat. Sear chicken for 5–6 minutes per side, or until golden and cooked through. Remove from pan and set aside.
Make the Dijon Cream Sauce:
In the same skillet, reduce heat to medium-low. Add minced garlic and sauté 30 seconds. Stir in chicken broth, scraping up browned bits. Whisk in heavy cream, Dijon mustard, grainy mustard (if using), and thyme. Simmer for 2–3 minutes.
Thicken if Needed:
If sauce is too thin, stir in cornstarch slurry and simmer another minute until thickened.
Combine and Serve:
Return seared chicken to the pan and spoon sauce over it. Add roasted potatoes. Simmer everything together for 2–3 minutes until well coated and heated through.
Garnish and Serve:
Sprinkle with chopped parsley and serve warm. Pairs well with steamed veggies or crusty bread.