Heat oven to 400°F Toss potatoes with olive oil and season with 1/4 teaspoon of the garlic salt and 1/8 teaspoon of the black pepper. Place on a baking sheet; bake at 400F for 30 minutes. Turn halfway through.
While potatoes are cooking, heat broiler or grill. Season steak with remaining garlic salt and pepper. Broil or grill steak 4 minutes per side for medium rare. Allow to rest before slicing.
Melt butter in a medium-size skillet over medium-high heat. Add shallots and mushrooms and cook 6 minutes, stirring occasionally. Add sherry off the heat. Return to heat; cook for 1 minute. In a small bowl, mix together broth and cornstarch. Stir into skillet and simmer for 2 minutes or until thickened. Stir in oregano and Worcestershire sauce. To serve, slice steak on an angle; spoon mushroom sauce on top. Serve with potatoes.