Splash the olive oil into two sauté pans, have at least one lid on hand for the cauliflower dish. Place the pans each on medium-low heat.
Toss the onions into one, the cauliflower into the other. Toss lightly and then reduce the heat to low.
Cover the cauliflower with the lid. This will cause any liquid that is seeping out of the cauliflower to stay around a bit longer to steam the florets into tenderness.
Toss each vegetable lightly until golden. It will take awhile, about 15-25 minutes. If they are browning to quickly, reduce the heat or add a splash of water, which will steam them lightly and prevent burning.
When the vegetables are nearly perfect, toss them together in the same sauté pan.
Add the balsamic vinegar to the empty pan and increase the heat to high. Take care not to inhale any of the vinegar fumes.
As soon as all of the vinegar is bubbling up. Turn off the heat. Toss the cauliflower and onions into the balsamic with the salt, coating thoroughly. Add more salt to taste, if desired.
Serve hot, cold or room temperature.