In a large bowl, combine the lemon juice, 3 tablespoons of the oil, ½ teaspoon salt, and ¼ teaspoon pepper. Add the mushrooms, cheese, and parsley and toss.
Cut the steak into pieces as necessary to fit in a large skillet. Season with ½ teaspoon each salt and pepper.
Heat the remaining oil in the skillet over medium-high heat. Cook the steak to the desired doneness, 4 to 5 minutes per side for medium-rare.
Slice the steak and serve with the mushroom salad.
