Melt the unsalted butter and olive oil in a large 12-inch skillet or Dutch oven over medium heat. Add the thinly sliced sweet onions and fine sea salt.

Cook, stirring every few minutes, for 10 minutes until the onions become soft, translucent, and begin turning pale gold.

Reduce heat to medium-low. Continue cooking for another 20 to 25 minutes, stirring frequently and scraping the browned fond off the bottom of the pan, until the onions reduce down into a deep, jammy amber caramel. If the pan gets dry, splash in 1 tablespoon of water or broth to deglaze.

Stir in the minced garlic and fresh thyme leaves during the final 2 minutes of cooking until fragrant. Remove from heat and let cool for 5 minutes.

Preheat your oven to 375°F (190°C). Lightly grease an 8-inch cast iron skillet or 1.5-quart ceramic baking dish.

In a large mixing bowl, beat the softened cream cheese, sour cream, mayonnaise, Worcestershire sauce, and black pepper with a spatula or hand mixer until completely smooth and fluffy.

Fold in 1 cup of the shredded Gruyère cheese, all of the sharp white cheddar, and the cooled caramelized onions until evenly distributed.

Spread the dip evenly into the prepared skillet or baking dish. Top evenly with the reserved 1/2 cup of shredded Gruyère cheese.

Bake at 375°F (190°C) for 20 to 22 minutes until the edges are vigorously bubbling and the cheese is hot and molten.

Switch the oven to broil on HIGH for 2 to 3 minutes, watching closely, until the top cheese layer turns deeply golden-brown with blistered crispy spots.

Remove from the oven, garnish with fresh thyme leaves and cracked black pepper. Let rest for 5 minutes before serving hot with crisp toasted baguette crostini, pita chips, or fresh vegetables!