01 - Preheat the oven to 425°F. Toss the halved baby potatoes with 1 tbsp olive oil, paprika, garlic powder, half the thyme, and a generous seasoning of salt and pepper. Spread them in an even layer on a baking sheet.
02 - Roast the potatoes for 30–35 minutes, turning them halfway through, until they are deeply golden and crisp on the edges. Remove from the oven when done.
03 - Season the chicken breasts on both sides with salt and pepper. Heat the remaining 1 tbsp olive oil and the butter in a large skillet over medium-high heat. Sear the chicken for 4–5 minutes per side until a golden crust forms and the meat is nearly cooked through. Transfer to a plate and tent loosely with foil.
04 - In the same skillet, cook the chopped onion over medium heat until softened, about 2–3 minutes. Add the minced garlic and stir briefly. Add the sliced mushrooms and the remaining thyme; cook for 5–6 minutes, stirring occasionally, until the mushrooms are well browned and their moisture has evaporated.
05 - Pour in the chicken stock, scraping up any browned bits from the bottom of the pan. Let it simmer for 2 minutes to reduce slightly. Lower the heat and stir in the heavy cream. Return the seared chicken breasts to the skillet and simmer gently for 5 minutes, or until the chicken is cooked through and the sauce has thickened. If a thicker sauce is desired, dissolve the cornstarch in 2 tbsp of water and stir it into the skillet; cook for 1–2 minutes more.
06 - Stir the chopped parsley into the sauce. Plate each chicken breast with a generous spoonful of the creamy mushroom sauce and arrange the crispy herb potatoes alongside. Garnish with additional fresh thyme sprigs and serve immediately.