01 - Preheat the oven to 425°F. Toss halved baby potatoes with 1 tbsp olive oil, paprika, garlic powder, half the thyme, and a generous pinch of salt and pepper until evenly coated.
02 - Spread the seasoned potatoes in a single layer on a baking sheet. Roast for 30–35 minutes, turning halfway through, until deeply golden and crisp on the edges.
03 - Season the chicken breasts on both sides with salt and pepper. Heat the remaining 1 tbsp olive oil and 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 sauté briefly until fragrant. Add the sliced mushrooms and remaining thyme; cook for 5–6 minutes, stirring occasionally, until the mushrooms are deeply browned and their moisture has evaporated.
05 - Pour in the chicken stock, scraping up any browned bits from the bottom of the pan. Simmer for 2 minutes to reduce slightly. Lower the heat and stir in the heavy cream. Return the seared chicken to the skillet and simmer gently for 5 minutes, or until the chicken is cooked through and the sauce has thickened. For a thicker sauce, whisk the cornstarch with 2 tbsp cold 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 serve alongside the crispy herb-roasted potatoes. Garnish with additional fresh thyme.