Very simple, but the sauce is so delicious! I don’t know what Mexican seasoning is, so I just leave it out, or throw in a little cumin and chili powder.
Combine orange juice, cilantro and Mexican seasoning in small bowl. Place steak and 1/3 cup orange juice mixture in a Ziploc bag; turn steak to coat. Marinate in refrigerator 6 hours or as long as overnight, turning occasionally.
Combine remaining orange juice mixture with sour cream and green onion. Cover and refrigerate until ready to serve.
Remove steak from marinade; discard marinade. Place steak on grill over medium heat, and cook until desired doneness.
Let rest for 10 minutes, then carve steak diagonally across the grain into thin slices; season with salt and pepper as desired. Serve with citrus sauce.