2. Peel and slice the onions 1/2" thick. Rub the onion slices (trying to keep them intact) with 1 to 2 teaspoons of olive oil and sprinkle with salt and pepper.
3. Grill the onions until they are lightly charred and cooked through, about 3 to 4 minutes per side.
4. 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.)