Caramelize the onions.

Melt butter in a large skillet over medium heat. Once melted, add onions, sugar, and salt and cook, stirring occasionally (every few minutes) until onions have begun to reduce in volume.

Once onions are softened and turning translucent continue to cook, stirring occasionally/as needed (keep from burning) until onions are caramelized and deep golden brown in color. If the onions begin to stick, add a splash of water or broth to deglaze the pan.

Once caramelized, remove onions from pan and allow to cool completely before proceeding with the recipe (you may cover them and refrigerate to speed up the cooling process).

Make the dip

In a large mixing bowl, stir together sour cream, mayo, onion powder, and salt until well-combined. Add (cooled) onions and stir well.

Cover and refrigerate for at least 1 hour before serving. Flavor develops as dip sits.