Makes about 2 1/2 cups
2 tablespoons unsalted butter
1 red onion, chopped
1/2 yellow onion, chopped
1 shallot, minced
1 garlic clove, minced
1/4 teaspoon kosher salt, plus more to taste
Pinch freshly ground black pepper, plus more to taste
2 cups sour cream
2 teaspoons Worcestershire sauce
2 teaspoons soy sauce
Cut a piece of parchment paper to fit inside either a 12-inch skillet or a 4-quart saucepan, whichever you prefer for caramelizing onions. Set the paper aside.
Over low heat, melt the butter in your preferred skillet or saucepan. Add the red and yellow onion, shallot and garlic. Sprinkle the mixture with 1/4 teaspoon of salt and a pinch of pepper; stir to combine. Lay the cut parchment directly over the onion mixture. Place a lid on the pan. Cook the mixture until the onions are very soft and beginning to color, about 20 minutes.
Remove the lid and parchment. Increase the heat slightly and continue cooking until the mixture is deeply caramelized, about 30 minutes. Taste and adjust the seasoning with additional salt and pepper. Allow the mixture to cool to room temperature.
In a large bowl, combine the sour cream, Worcestershire sauce, soy sauce and caramelized onion mixture. Season with additional salt and pepper to taste. Chill the dip until ready to serve.