In a shallow glass or ceramic dish, combine garlic, orange rind, juice, vinegar, pepper, mustard, and mint. Add steak to marinade; turn once to coat. Cover with plastic wrap and refrigerate for at least 4 hours, turning steak twice in marinade. Remove steak from marinade, scraping any bits of marinade clinging to meat back into the bowl. Transfer marinade to small saucepan and bring to a boil; reserve.
Lightly grease grill rack with vegetable cooking spray. Preheat charcoal grill until coals have turned a gray ashy color, or preheat gas grill according to mfg's suggested time on high heat.
Place steak on grill 4 inches from heat source and sear 1 1/2 minutes on each side. Brush with a little reserved marinade and continue cooking, covered (with lid down or tented with foil), for approximately 4 minutes, brushing frequently with marinade. (Make a little slit in center of the steak to check for doneness. Steak should be pink inside.) Transfer to carving board, tent with foil, and let rest for 7 minutes before slicing.
Arrange orange slices in overlapping pattern around outside of platter.
Slice steak diagonally across the grain into very thin slices. Arrange down the center of the platter and garnish with mint.