01 - Preheat the oven to 425°F. Line a large rimmed baking sheet with parchment paper.
02 - Toss the halved baby potatoes with 1 tablespoon olive oil, paprika, garlic powder, half of the thyme, and a generous pinch of salt and pepper. Arrange cut-side down on the prepared baking sheet. Roast for 30–35 minutes, flipping halfway through, until deeply golden and crisp on the edges.
03 - Pat the chicken breasts dry and season both sides with salt and pepper. Heat the remaining 1 tablespoon olive oil and the butter in a large skillet over medium-high heat until the butter foams. Sear the chicken for 4–5 minutes per side until a golden crust forms and the internal temperature approaches 155°F. Transfer to a plate and tent loosely with foil.
04 - In the same skillet, reduce the heat to medium. Add the chopped onion and cook until softened, about 2–3 minutes. Stir in the minced garlic and cook for 30 seconds until fragrant. Add the sliced mushrooms and the remaining thyme; cook undisturbed for 2 minutes, then stir occasionally for another 3–4 minutes until the mushrooms are deeply browned and their moisture has evaporated.
05 - Pour the chicken stock into the skillet, using a wooden spoon to scrape up any caramelized bits from the bottom. Simmer for 2 minutes to reduce slightly. Lower the heat and stir in the heavy cream. Return the seared chicken breasts and any accumulated juices to the pan. Simmer gently for 5 minutes, turning the chicken once, until the internal temperature reaches 165°F and the sauce coats the back of a spoon. If a thicker sauce is desired, drizzle in the cornstarch slurry while stirring and cook for 1–2 additional minutes.
06 - Remove the skillet from the heat and fold in the chopped parsley. Arrange each chicken breast on a plate, spoon the creamy mushroom sauce generously over the top, and serve alongside the crispy herb-roasted potatoes. Garnish with fresh thyme sprigs.