London Broil, Aged Cheddar and Watercress on Croissant with Horseradish Mayonnaise
Heat a grill pan or griddle over medium heat. Add 3 slices of beef in an even layer next to each other, and cook for 30 seconds per side, just to warm through. Top the slices 1 or 2 slices of the cheese, cover with a lid and let melt, about 20 seconds. Spread some of the mayonnaise on both sides of each croissant. Layer on beef, cheese, onion and watercress.
*To make with flank steak: Instead of London broil or roast beef, use 1 1/4 pound flank steak, trimmed of excess fat. Let marinate in olive oil, chopped garlic and fresh thyme for at least 2 hours and up to 24 hours. Heat a grill to high or a grill pan over high heat until smoking. Season the steak with salt and lots of black pepper, and cook on both sides until golden brown and slightly charred and cooked to medium-rare doneness, about 6 minutes per side. Remove from the heat and let rest 10 minutes before slicing into thin slices against the grain of the meat.