How to Make It
Heat the olive oil in a large skillet until almost smoking. Add the mushrooms and cook over moderately high heat, stirring, for 2 minutes. Stir in 1 tablespoon of the butter, reduce the heat to low and cook until the mushrooms are browned and the pan is dry, about 7 minutes. Season with salt and pepper and transfer to a saucepan.
Melt 2 more tablespoons of the butter in the skillet. Add the leeks, cover and cook over low heat, stirring a few times, until tender, about 10 minutes. Season with salt and pepper and add to the mushrooms along with the chicken stock; simmer for 3 minutes. Season with salt and pepper.
Preheat the oven to 450°. In a large, heavy-bottomed ovenproof skillet, heat the vegetable oil until almost smoking. Season the fish with salt and pepper and add to the skillet, skin side down. Shake the skillet to loosen the fillets and cook over high heat until the skin is browned and crisp, about 5 minutes. Turn the fillets, transfer the skillet to the oven and roast for about 4 minutes, or until the fish is just cooked through.
Stir the butter pieces and chopped parsley into the mushroom-leek sauce. Divide the sauce among 4 large plates and set the bass fillets on top. Serve at once.