Heat potatoes and enough water to cover to boiling in small saucepan over high heat. Reduce heat to low; cover and simmer 15 minutes or until potatoes are tender. Drain well.
Melt 2 tablespoons butter in 12-inch skillet (with ovenproof handle) over medium heat. Stir in leeks. Cook 5 minutes or until leeks are tender.
Preheat oven to 350º F.
Combine eggs, milk, TABASCO® Sauce, rosemary, and salt in medium bowl. Stir in potatoes and sliced flank steak. Swirl remaining 2 tablespoons butter over bottom and side of skillet. Pour in egg mixture; cook over very low heat, shaking pan back and forth, until frittata is firm on bottom and almost set, about 8 to 10 minutes. Remove from heat; sprinkle with cheese.
Bake 5 to 8 minutes or until eggs are set and cheese is melted.
Makes 6 servings.