01 - Preheat the oven to 425°F. In a mixing bowl, toss the halved baby potatoes with 1 tbsp olive oil, paprika, garlic powder, half of the thyme, and a generous pinch of salt and pepper until evenly coated.
02 - Spread the seasoned potatoes in a single layer on a large baking sheet. Roast for 30 to 35 minutes, flipping halfway through, until the edges are deeply golden and crisp.
03 - Pat the chicken breasts dry and season 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 to 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 for 2 to 3 minutes until softened. Add the minced garlic and stir briefly. Add the sliced mushrooms and the remaining thyme; cook for 5 to 6 minutes, stirring occasionally, until the mushrooms are deeply browned and their moisture has evaporated.
05 - Pour the chicken stock into the skillet, scraping up any browned bits from the bottom. Let it simmer for 2 minutes. Reduce the heat to low and stir in the heavy cream. Return the seared chicken breasts to the pan along with any accumulated juices. Simmer gently for 5 minutes until the chicken is cooked through and the sauce has thickened. If a thicker sauce is desired, stir in the cornstarch slurry and cook for 1 to 2 additional minutes.
06 - Remove 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 roasted potatoes. Garnish with fresh thyme sprigs.