Sweet onions are just what they sound like - an onion with a milder flavor due to a higher sugar content. Popular varieties of sweet onion include Vidalia and Walla Walla.    Some people claim you can eat a sweet onion raw like an apple. I’ve never tried that, but I do enjoy sweet onions in this feta and tomato salad. The relatively mild flavor of a sweet onion means you can eat them raw in this recipe without the typical sharp, pungent taste of a regular onion.