Preheat grill. Meanwhile, prepare the steaks. Trim any excess fat or connective tissue from them. One at a time, place each steak piece between two sheets of waxed paper; pound with a meat pounder until they are flattened to a thickness of about 1/2".
Press the garlic cloves through a garlic press and put into a large, shallow dish. Add the orange and lemon juices and the olive oil and stir well. Add the steaks, turning them in the mixture to coat them, and leave them in the dish at room temperature.
To prepare the Orange-Onion Salsa: Segment the oranges. Using a sharp knife, first cut off the peel at the top and bottom of each orange, cutting thickly enough to remove the outer membrane of the orange segments as well. Then stand each orange on a cut end and cut off the peel in strips in the same way.
Holding the peeled orange over a bowl to catch the juices, free each segment by cutting between it and the membranes on either side; cut the freed segment crosswise in half; let the halves drop into the bowl. Add the sliced onion and cilantro and toss well, seasoning with salt and pepper to taste.
Carefully oil the grill rack. Remove the steaks from their marinade, pat them dry, and season them with salt and pepper to taste. Place the steaks on the rack and cook, turning once, until done to your liking, 12-15 minutes total for medium.
Serve the steaks, passing around the Orange-Onion Salsa for guests to spoon alongside the meat.