03Heat 2 tablespoons oil in large nonstick skillet over medium heat. Mound potato mixture in skillet in 2 portions, dividing evenly. Using metal spatula, press each into 5-inch oval. Cook until bottom is brown and crisp, about 10 minutes. Turn and cook until second side browns, about 5 minutes. Transfer galettes to baking sheet. Keep warm in oven.
04Heat 1 tablespoon oil in heavy medium skillet over medium-high heat. Sprinkle steaks with salt and pepper. Add to skillet; cook to desired doneness, about 4 minutes per side for medium-rare. Transfer steaks to plate (do not clean skillet). Add sauce to skillet and bring to boil, scraping up browned bits. Mix in mustard. Remove from heat. Add remaining 2 tablespoons butter and whisk just until melted.
05Place potato galettes on plates. Top each with 1 steak. Spoon sauce over. Sprinkle with remaining 1 tablespoon parsley and serve.