For the best results, ensure the sirloin steak is thinly sliced against the grain for maximum tenderness. American cheese is essential for that authentic Philly Cheesesteak flavor, but feel free to experiment with other melting cheeses like provolone or white cheddar. If you prefer a thicker sauce, whisk in an additional teaspoon of flour during step 5. For an extra touch of flavor, consider adding a pinch of red pepper flakes to the beef mixture.