01 - Preheat the oven to 425°F (220°C). Position the rack in the center.
02 - Toss the halved baby potatoes with 1 tablespoon olive oil, paprika, garlic powder, half the thyme, and a generous pinch of salt and pepper. Spread in an even layer on a baking sheet. Roast for 30–35 minutes, flipping halfway through, until the potatoes are deeply golden and crispy on the edges.
03 - Season the chicken breasts on both sides with salt and pepper. Heat the remaining 1 tablespoon olive oil and butter in a large skillet over medium-high heat. Once the butter is foaming, 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, stirring occasionally, until the mushrooms are deeply browned and their moisture has evaporated, about 5–6 minutes.
05 - Pour in the chicken stock, using a wooden spoon to scrape 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 and any accumulated juices to the skillet. Simmer gently for 5 minutes, or until the chicken is cooked through (internal temperature reaches 165°F) and the sauce has thickened to a silky consistency. For a thicker sauce, whisk the cornstarch with 2 tablespoons of cold water and stir it into the skillet; cook 1–2 minutes more until glossy and coats the back of a spoon.
06 - Remove from heat and fold in the chopped parsley. Plate each chicken breast over a bed of the creamy mushroom sauce, accompanied by the crispy herb potatoes. Garnish with additional fresh thyme sprigs and a crack of black pepper.