Slice the 3 medium to large sized onions take care to keep them all the same size
Heat the cast iron casserole dish over medium high heat
Add the 6 Tbs butter and let it melt
Scrape the onions into the melted butter (do not cover the casserole dish)
Add the 1/2 Tsp salt
Stir the onions distributing the salt
Lower the heat and let them become translucent
Once they are translucent, bring the heat to very low. Over the next 50-55 minutes is where utilizing your patience becomes important. These are going to smell so good, you will be tempted to not let them caramelize all the way.
Stir the onions occasionally in the first 20 minutes and a bit more often as they begin to brown. They are done when they turn a brown caramel color. When caramelizing onions some people cook them until the pan is brown and they have to scrape it. That would be fine for a different dish however for our dip we took the onions to the color of caramel.
Some of the oniony goodness will accumulate on the sides and the bottom, make sure you scrape all of this including any butter along with your caramelized onions into a mixing bowl.
Let the onions come to room temperature
Add the 1/4 Tsp onion powder and the 8 Oz. of sour cream and mix it all together
Place it into the refrigerator for at least one hour - The longer it sits the more flavor it will develop. This can be made a day ahead to achieve the maximum flavor.