In a large glass measuring cup or small mixing bowl, combine the Ponzu sauce with ¼ cup oil, the sesame oil, ginger, garlic, and red pepper flakes. Pour ¾ cup of the marinade into a large resealable food storage bag. Add the honey to the rest of the marinade and reserve for later. Add the steak to the bag, squeeze out as much air as possible, seal the bag, and massage the steak a bit to get the marinade all over it.
Let the steak sit at room temperature for a minimum of 15 minutes and up to 30 minutes or in the refrigerator for up to 8 hours. If refrigerating, remove from the fridge 30 minutes before cooking to allow the meat to come to room temperature.
If cooking on a grill, preheat the grill to medium-high. Oil the grates well.
Remove the meat from the marinade, discarding the marinade. Pat the meat dry on both sides. Grill meat for 4-6 minutes per side for medium rare or until it reaches an internal temperature of 125-130. Remove the steak from the pan to a cutting board, cover loosely with foil, and let rest for 10 minutes.
Pour the reserved marinade into a small saucepan and bring to a boil over medium heat. Boil for about 2 minutes or until the sauce begins to thicken.
Slice the steak across the grain, put on a serving platter, spoon the sauce over the top and top with the green onions.
If cooking on a stove top, heat a grill pan or cast iron skillet over medium-high heat. Add 1 tablespoon oil and heat until it starts to shimmer. Add the steak and cook 4 minutes, flip and cook another 4-5 minutes or until it reaches an internal temperature of 125-130 degrees for medium rare. Remove the steak from the pan to a cutting board, cover loosely with foil, and let rest for 10 minutes.
Pour the reserved marinade into a small saucepan and bring to a boil over medium heat. Boil for about 2 minutes or until the sauce begins to thicken.
Slice the steak across the grain, put on a serving platter, spoon the sauce over the top and top with the green onions.