2. Meanwhile, peel and slice the onions 1/2" thick. Rub the onion slices (trying to keep them intact) with the olive oil and sprinkle with salt and pepper.
3. Preheat the grill to medium-high heat.
4. Grill the onions until they are lightly charred and cooked through, about 3 to 4 minutes per side.
5. When the onions are cool enough to handle, chop them roughly and put then in a bowl. Add the balsamic vinegar. (This can be stored in the refrigerator for up to 1 week.)
6. Grill the flank steak for 4 to 8 minutes on each side, depending on the desired doneness. Let the steak rest on a carving plate for 1 or 2 minutes.
7. Slice the steak on the bias and serve it with the onion relish.