In a cast-iron skillet, melt 2 tablespoons of butter and 2 tablespoons of olive oil over medium-low heat. Add 3 sliced yellow onions, stirring to coat. Cook for about 30-40 minutes, stirring occasionally until the onions are golden brown and jammy.

Once the onions are caramelized, transfer them to a mixing bowl and let them cool slightly. Add 8 ounces of softened cream cheese, 4 ounces of Brie, and 4 ounces of shredded Gruyere cheese. Stir in 2 minced garlic cloves and 1 tablespoon of fresh chopped thyme.

Preheat your oven to 350°F (175°C). Pour the dip mixture into a baking dish. Cook in the oven for 30-40 minutes until the dip is bubbly and heated through.

Once the dip is hot, sprinkle additional Gruyere cheese on top and broil for a few minutes until golden and bubbly. Serve warm with dippers.