To prepare the beef, combine first 5 ingredients, and rub evenly over beef. Cover and chill the beef for 2 hours.
To prepare gratin, preheat oven to 400°.
Arrange potato slices in a single layer on a jelly roll pan coated with cooking spray; coat tops of slices with cooking spray. Bake at 400° for 15 minutes or until soft. Set aside.
Heat a large nonstick skillet coated with cooking spray over medium heat. Add onion; cook 10 minutes or until lightly browned, stirring frequently. Stir in broth and next 5 ingredients (broth through 1/4 teaspoon pepper); cook 3 minutes, stirring occasionally. Remove from heat.
Add cream cheese to onion mixture; stir until well blended. Stir in sour cream. Add potato slices to pan; stir gently to combine. Place mixture in a shallow 1-quart casserole coated with cooking spray. Combine breadcrumbs and Parmesan cheese, and spoon evenly over onion mixture. Lightly coat surface of gratin with cooking spray. Bake at 400° for 30 minutes or until golden brown.
Place beef on broiler pan coated with cooking spray; cook 3 minutes on each side or until desired degree of doneness. Cut steak diagonally across grain into thin slices.