1. Light a grill or heat a grill pan. Set the steak flat on a work surface. Using a sharp knife, carefully cut through the center of the steak (parallel to the work surface), leaving 1/2 inch of the meat attached at the side so it can be opened like a book. Set the butterflied flatiron steak in a medium bowl or baking dish.
2. In a blender, combine the orange juice with the chipotle, adobo, garlic and 1 tablespoon of the olive oil and puree until smooth. Pour the marinade over the steak and let stand for 10 minutes.
3. Meanwhile, in another medium bowl, gently mix the avocados with the corn, red onion, jalapeño, lime juice and the remaining 1 tablespoon of olive oil. Season with salt and pepper.
4. Remove the steak from the marinade, letting the excess drip back into the bowl; do not wipe off the marinade. Season the steak with salt and pepper. Grill the butterflied steak flat over moderately high heat, turning once, until medium, about 6 minutes on each side. Transfer the steak to a cutting board, cover with foil and let rest for 5 minutes. Thinly slice the steak across the grain and serve with the avocado-corn relish.