Slice four large yellow onions in half lengthwise, lay flat, and slice thinly into half-moon shapes.

Heat a large heavy-bottomed skillet over medium heat, add 4 tablespoons of unsalted butter, and let it melt and foam slightly.

Add the sliced onions to the melted butter and stir to coat evenly.

Reduce heat to medium-low and cook the onions undisturbed for 5 minutes, then stir every 3–4 minutes for a total of 25–30 minutes until they turn golden to deep amber.

When onions are light golden (around 20 minutes), add 4 minced garlic cloves and cook for 2–3 minutes until fragrant.

Continue cooking and stirring the onion mixture for another 8–10 minutes until the onions are jam-like and deep amber.

Remove the skillet from heat and let the caramelized onions cool for 5 minutes.

In a medium mixing bowl, whisk together 1 cup mayonnaise, 1/2 cup sour cream, 2 tablespoons ketchup, 1 tablespoon horseradish, and 1 teaspoon Worcestershire sauce until smooth.

Add 1/2 teaspoon smoked paprika, 1/4 teaspoon cayenne pepper, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, salt, and black pepper, then whisk until spices are evenly distributed.

Fold the cooled caramelized onions and garlic into the creamy base until no white streaks remain.

Taste and adjust seasoning if needed, then refrigerate for at least 2 hours or overnight before serving.