Lightly toast bread. Cut off crusts; cut slices in half diagonally.
Heat butter in large skillet over medium-high heat until lightly browned. Add garlic and sirloin; saute until beef is cooked through, 5 minutes. Add sherry; cook 1 minute.
Mix broth, flour, salt, pepper and mustard in bowl. Add to skillet; cook 2 minutes to thicken.
Serve beef and sauce with toast.