Heat a large nonstick skillet or pot over medium heat. Once hot, add the butter. Once the butter has melted, swirl to coat the bottom of the pan.
Add the onions and fine sea salt, turn the heat down to medium-low, and cook for about 30 minutes to 1 hour (or until they have caramelized). Stir them every now and then, and add a Tablespoon or two of water if they ever look like they are drying out. This process takes a little patience. You can cook it on a higher temperature if you’re really pressed for time, but the flavor won’t be as good. If you have the time I highly recommend cooking it low and slow.
Once the onions are caramelized, remove the pan from the heat, and let them rest for about 15 minutes, or until they’ve cooled slightly.
To a medium mixing bowl, add the sour cream, mayonnaise, onion powder, garlic powder, and caramelized onions. Mix everything together.