Using a VERY sharp knife slice the onions as thin as you are able.
Place the sliced onions into a bowl and set aside.
Use only FRESH lemon juice. Usually 2 medium lemons will have plenty of juice. Squeeze the lemons until you have 1/4 cup juice. Discard the seeds and pour juice into a small bowl. Add the vinegar to the lemon juice.
Sprinkle the onions with salt and red pepper to taste.
Pour the lemon/vinegar mixture over the onions and gently toss with your fingers until all the onions are wet and the salt and red pepper is evenly distributed.
Cover the bowl and allow to sit at room temperature for about 45 minutes. The onion will turn bright pink and decrease in volume.
Taste an onion slice. When it's zingy and crunchy and yummy serve with your favorite dish.