Melt 3 Tablespoons butter in a large skillet over medium heat. Add the onions, salt & pepper. Sauté, stirring regularly, until the onions begin to brown. Reduce the heat to low and continue cooking 40-45 minutes, stirring occasionally, until onions are deep brown and caramelized.
Stir in cooking sherry, Worcestershire sauce, onion powder, and garlic powder. Set aside to cool slightly.
In a medium mixing bowl, combine the sour cream, mayo, and caramelized onions. (Reserve some onions for garnish.) Mix well.
Transfer to a small serving bowl; garnish with reserved onions, fresh chives, and fresh cracked pepper.