On a cutting board, trim the root end, and tip of the onion off. Half each onion and remove the thick, papery outer skin layer. Thinly slice each half from top to bottom. My onions were about 1/4" thick- you can go thinner. I love onions so mine were on the thicker side.
Add 2 tablespoons of butter to a large skillet and melt it over low heat.
Add onion slices into the pan, and mix to coat with butter.
Over low heat, cook the onions with a cover, stirring every 5 minutes. They will start to break down and get soft, translucent, and take on a nice deep coloring as they cook. Once they have cooked for 15 minutes, add the salt, sugar, and vinegar to the pan. Cook onions for an additional 15 minutes. Note: These are lightly caramelized, if you want a deeper amber colored onion, caramelize for the full 45 minutes over low heat. For this recipe, they will be golden colored, and soft in texture at the 30-minute mark.
Allow onions to fully cool in the pan before adding them to the cold ingredients.
In a large mixing bowl, add cooled onions to the sour cream, and mayonnaise. Sprinkle with garlic powder, and black pepper. Stir to combine dip fully.
Cover the bowl with plastic wrap and chill dip for at least 4 hours, or up to 24 hours to allow the flavors to meld. Serve with thick potato chips. This is a chunky dip so ruffled chips work best. Sprinkle with chopped parsley for garnish *optional.