In a skillet over medium heat, melt 2 tablespoons of the butter and add the sliced onion. Cook until the onion is caramelized, about 45 minutes, stirring occasionally.

In a bowl, combine the caramelized onions, remaining butter, salt, black pepper, minced garlic, Worcestershire sauce, dried thyme, and chopped parsley. Mix until well combined.

Transfer the mixture onto a piece of parchment paper and shape it into a log. Wrap tightly and refrigerate until firm.