Pour 1 cup of white vinegar into the saucepan at the same time as you prepare onions in the kitchen.
Place the saucepan onto the stove top and set the burner to medium-high.
Bring the vinegar to a boil while you saute the onions. Reduce the temperature of the vinegar to medium to keep it at a low boil while the onions cook.
Remove the vinegar from the heat when you finish cooking the onions. Discard the vinegar after you incorporate the onions to the recipe you are making.