Rinse cauliflower and broccoli and cut into florets. Place in a large mixing bowl.
Add chopped onion to veggie mixture.
In a small bowl, combine 1/2 cup mayonnaise, vinegar and sugar; stir well.
Add Ranch dressing mix to the mayonnaise mixture and stir well to combine.
Pour dressing mixture over the veggies; toss until well-coated. Add more mayonnaise, if needed, to just lightly coate the veggies...too much will make the salad soggy.
Refrigerate several hours, or overnight, before serving.
An easy, simple side dish for Spring and Summer potlucks. Try adding in chopped ham and serve as a refreshing luncheon salad. Enjoy!