Melt butter in a skillet over medium-high heat. Add the sliced onions to pan, stirring occasionally. When the onions start to brown, lower heat and continue to cook on medium-low heat until onions are caramelized, or about 20-25 minutes.
Add cream cheese, sour cream, mayonnaise, and seasoning to food processor and blend until smooth.
When onions are cool, add to the dip in the food processor and pulse, just enough to mix in the onions. Scoop the dip into a serving bowl.
Notes
Nutrition information is provided as an estimate only.