In a large, heavy, self-sealing plastic bag, combine lemon juice, oil, oregano, pepper and garlic; shake to blend well. Place steak in bag; turn to coat steak on all sides. Refrigerate for at least 3 hours or up to 24 hours.
About 15 - 20 minutes before cooking steak, remove it from bag and set aside; reserve marinade. Just before cooking steak, combine tomatoes and onions in a small bowl; set aside.
Preheat an 11-in. square or 12-in. round grill pan over medium-high heat until rim of pan is hot to the touch. Rub pan with about 1 tsp. butter and wait for about 1 more minute. Place steak in pan and cook until well browned on bottom, about 6 minutes; turn steak and reduce heat to medium. Continue to cook until steak is well browned on second side and done to your liking; cut in thickest part to test, about 6 more minutes for medium-rare.
Meanwhile, heat a 10-in. omelette pan or 3-qt. sauté pan over medium-high heat until rim of pan is hot to the touch. Add remaining 5 tsp. butter and wait for about 1 more minute. Add mushrooms and cook, stirring often, until tender and lightly browned, about 5 minutes. Add reserved marinade; cook until almost all liquid has evaporated.
Place steak on a serving board or platter; cut across the grain into thin slanting slices. Spoon mushrooms over servings of steak; season to taste with salt. Garnish with tomato mixture.