Combine the orange juice, lime juice, olive oil, minced garlic, chipotle chilies, cilantro, cumin, salt, brown sugar and black pepper in a blender. Puree until smooth.
Place the flank steak in a large zip plastic bag. Pour the marinade over the steak. Refrigerate for two to four hours.
Drain the marinade from the beef and discard.
Lightly oil the grill or grill pan. Grill the steak for four to five minutes on medium-high heat on each side. Transfer to a cutting board and let rest.
Meanwhile, return the grill pan to medium heat and add the peppers and onions. Cook for three to five minutes on each side, until they start to brown. Flip and cook until just wilted.
Slice the steak against the grain, diagonally, into strips.
To build your fajita, add a couple strips of steak to a tortilla, top with peppers, onions, avocado slices, salsa, sour cream, pico de gallo and/or cheese.