In a bowl, whisk together the orange juice, lime juice, soy sauce, brown sugar, ginger, and garlic. Whisk until sugar is dissolved. Continue whisking while slowly drizzling in the oils until combined. Alternately, you could vigorously shake all the ingredients together in a mason jar.

Remove steak from fridge about 30 minutes prior to cooking and leave at room temperature. Heat outdoor grill to med-high heat. If an outdoor grill is not available, use the broiler, or an indoor grill pan. Remove steak from bag and let excess marinade drip off.  Sprinkle steak lightly with salt and pepper.  Cook for about 5-8 minutes on each side or until it reaches an internal temperature of about 135 for medium rare. (In order to stay tender, flank steak should still be pink in the middle, but nicely seared on the outside.) When done, remove from grill and wait at least 5 minutes before cutting to let the juices redistribute. While steak is cooking and resting, prepare other salad ingredients.

Divide lettuce onto plates or bowls and top with slices of mango, avocado, and red onion.