Start by heating the unsalted butter in a large skillet over medium heat. Add the chopped sweet onions and a pinch of salt. Sauté the onions, stirring occasionally, until they are soft and caramelized, about 15-20 minutes.
In a mixing bowl, combine the softened cream cheese and mayonnaise. Use a spatula to mix until smooth.
To the creamy mixture, add the garlic powder, dried thyme, and salt. Stir well to combine. Then, fold in the shredded cheddar and smoked gouda cheeses until evenly distributed.
Once the onions have caramelized, let them cool for a minute. Then, fold the sautéed onions into the cheese mixture until everything is well combined.
Preheat your oven to 350°F (175°C). Transfer the mixture to a baking dish and spread it evenly. Bake for 25-30 minutes, or until the dip is bubbly and golden on top.
Remove the dip from the oven and let it cool slightly. Garnish with chopped parsley for a pop of color and freshness. Serve warm with crackers, bread, or fresh vegetables.