Remove sour cream from refrigerator until ready to use.
Melt the butter in a large saute pan over medium heat. Add the onions, salt, pepper, cayenne, and onion powder and cook for 2-3 minutes, then add the shallot and garlic for 2-3 more minutes.
Add the white wine and continue to cook, scraping the bottom of the pan with a wooden spoon to remove any brown bits.
Once the mixture has thickened slightly (should only take 2-3 minutes), reduce the heat and cook the entire mixture for about 20 more minutes, stirring often, until caramelized (soft, dark brown, and with a somewhat sweet flavor).
Remove from heat and stir in sour cream until well mixed. Refrigerate until ready to serve.
If possible, remove dip from refrigerator at least 20 minutes before serving (if not, it's still great cold!).
Garnish with sliced green onions and serve with chips or raw veggies.