Mix together all of the marinade ingredients: the orange juice, soy sauce, oil, sugar, mustard, garlic and red pepper flakes.
Put the flank steak into a ziplock bag then add the orange marinade, making sure the steak is coated in the sauce. Close the bag and refrigerate for at least 1 hour, but it’s best to let it sit at least 5 hours or overnight.
When the steak has been marinated and you are ready to cook, remove the steak from the fridge and let it sit at room temperature for about 20 minutes. While it’s sitting, you can make the salsa.
Making the salsa:
In a bowl combine the mango, pepper, cilantro and green onion.
In a separate bowl mix together the salsa dressing ingredients: lime juice, honey and curry paste. Then toss the dressing with the salsa mixture. Set aside.
Grilling the flank steak:
When you are ready to cook the steak, preheat your BBQ to high heat.
Brush the grill with a little bit of oil, then using tongs, remove the steak from the marinade and add the steak to the grill, searing the steak on both sides. Then turn down the heat to medium, and cook each side about 7 minutes, or longer if you prefer it more well done.
Remove the steak from the BBQ and let it rest about 10 minutes before you slice it up.
Putting the tacos together:
After the steak has rested, it’s time to slice it up. When slicing flank steak, make sure to cut it against the grain of the meat. For these tacos, slice the meat into thin strips.
Then assemble your tacos: Into a flour tortilla add the beef, then the salsa, chopped peanuts and fresh cilantro.