Melt 2 tablespoons butter and 1 tablespoon olive oil in a large skillet over medium heat. Add thinly sliced onions, salt, and fresh thyme. Cook slowly, stirring every few minutes, for 20 to 25 minutes until deeply caramelized, soft, and rich amber in color. Remove from heat and let cool for 5 minutes.

Preheat your oven to 375°F (190°C). Grease a 1-quart ceramic baking dish or 9-inch cast-iron skillet lightly with cooking spray.

In a large mixing bowl, beat softened cream cheese, sour cream, and mayonnaise together until completely smooth and lump-free.

Stir in 1 cup of shredded Gruyère, the white cheddar, Parmesan, minced garlic, Worcestershire sauce, Dijon mustard, and black pepper.

Fold in the caramelized onions, reserving 2 tablespoons for garnish. Mix until evenly incorporated.

Spread the dip evenly into the prepared baking dish. Top with remaining 1/2 cup of shredded Gruyère and reserved caramelized onions.

Bake for 20 to 22 minutes until heated through and bubbling around the edges. Turn on the broiler for the final 2 minutes until the top cheese is golden brown and toasted with bubbly spots.

Carefully remove from the oven. Garnish with fresh thyme leaves and let rest for 5 minutes before serving hot alongside toasted crostini and crisp apple wedges.