Preheat the oven to 425°. Arrange the mushroom slices on a large rimmed baking sheet in a slightly overlapping layer. Dot the mushrooms with the butter and drizzle with the chicken stock and olive oil. Season with salt and pepper. Roast the mushrooms in the center of the oven for about 50 minutes, turning occasionally, until the stock has evaporated and the mushrooms are tender and lightly browned in spots. Blot with paper towels and transfer to a plate. Sprinkle the mushrooms with the parsley and serve.