Heat the oil in a medium nonstick skillet over medium heat. Add the sweet onion and green onion (white part only) and cook, stirring often, until golden brown and soft, about 10 minutes. Allow to cool completely.
In a large bowl, stir together the onion mixture, yogurt and all remaining ingredients. Chill for 1 hour to let flavors meld (or up to 3 hours). Top with chopped green onions (green parts) just before serving. Serve with vegetables, crackers or potato chips.
Notes
If you are preparing this recipe as GLUTEN-FREE, just be sure to use a brand of mayonnaise that is known to be GF.