The one change I made was to use plain yogurt instead of sour cream. Make sure you caramelize the onions well - it makes their flavor mellow and it melts into the creamy mixture better.
And what if you have leftovers? Well, I did and decided to have some fun with them. I put the dip in the food processor with about 1 T of milk then the creativity flowed. I threw in some cilantro, spinach and arugula. Tasted, then added salt. Found a few crumbles of feta and invited them to the party. Then for the final piece of craziness I added about 3/4 of a fresh jalapeno. I tasted adjusted the green stuff where needed then took it to a football party complete with tortilla chips (and carrots, just in case there were any health nuts)
Caramelized Maui Onion Dip (Sunset Magazine)
2 tablespoons olive oil
2 Maui onions (or other sweet onions), halved and thinly sliced (about 5 cups)
1 cup sour cream
1/2 cup buttermilk
1 teaspoon kosher salt
1. Heat oil in a large nonstick frying pan over medium heat. Add onions and cook, stirring occasionally, until brown, sticky, and caramelized, about 20 minutes. If the pan starts to burn, reduce heat to medium-low and add 1 tbsp. water, stirring well. Transfer onions to a small bowl and chill until cold, about 1 hour.
2. Reserve 1 tbsp. onions. In a food processor, pulse remaining onions, sour cream, buttermilk, and salt just until combined. Transfer to a serving dish, top with reserved onions, and chill at least 1 hour.