In a small bowl, whisk the orange juice, lime juice, olive oil, brown sugar, soy sauce, cumin, coriander, garlic, and sea salt. Place the steak in a resealable plastic bag and pour in the marinade. Remove the air from the bag and seal it. Place it in your fridge and let it marinade for 3-24 hours. Note: I like to put a tea towel under the bag in case it leaks a little.
Preheat the grill to high. Make sure it is very very hot. Take the steak out of the marinade and place it on a plate and pour the marinade into a small pan. If you're using it, sprinkle the optional brown sugar over the steak. This will help it to char better.
Place the steak on the BBQ and grill for 3-4 minutes per side. Take the steak off and set it aside to rest. Slice thinly across the grain.
Toss the veggies in oil and BBQ until tender. Warm the tortillas on the grill.
While the veggies are grilling, bring the reserved marinade to a boil and boil rapidly until reduced to a thin syrup.
Pile the veggies and steak onto the tortillas, top with mango, avocado, feta, and a little drizzle of the reduced marinade.