In a large bowl, combine the lemon juice, 3 tablespoons of the oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Add the mushrooms, cheese, and parsley and toss to combine.
Cut the steak into pieces as necessary to fit in a large skillet. Season with 1/2 teaspoon salt and 1/2 teaspoon pepper. Heat the remaining 1 tablespoon of 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.