In a large skillet, heat two tablespoons olive oil over medium high heat. Add mushrooms and salt and pepper to taste. Saute mushrooms until they are brown, about 4 minutes. Transfer mushrooms to a plate.

Wipe the skillet with a paper towel. Heat remaining tablespoon of olive oil in the skillet, over medium high heat. Add the fish and season with salt and pepper, to taste. Cook fish for 3 minutes on one side. Do not move the fish. Flip the fish to the other side and cook for another 2 minutes, or until the fish is cooked through. Transfer the fish to a plate.

Heat the chicken stock over high heat, in the skillet. Reduce the broth by half. Remove skillet from heat. Whisk in the butter and lemon juice, until fully combined.

Place fish on individual plates, top with mushrooms and sauce. Serve hot.