Heat butter and olive oil in a large, heavy-bottomed skillet over medium heat until shimmering. Add sliced onions, salt, and black pepper, stirring occasionally to coat evenly with fat. Cook for 25-30 minutes, stirring every 3-4 minutes to ensure even caramelization without burning.

Watch for onions to transform from raw to golden, then deep amber brown with slightly crispy edges. The slower you cook them, the sweeter and more complex they'll become—this is where the magic happens with caramelized onion flavor.

Stir in minced garlic and fresh thyme during the final 2 minutes of onion cooking. This prevents garlic from burning while building layers of aromatic depth throughout the dip.

In a separate bowl, combine softened cream cheese, sour cream, and beef broth using an immersion blender until completely smooth. Fold in shredded gruyere cheese and grated parmesan until fully incorporated and melted.

Add the caramelized onions to the creamy caramelized french onion dip recipe base, stirring thoroughly to distribute them evenly. Season with cayenne pepper for a subtle heat that complements rather than overpowers the sweet onion notes.

Transfer the mixture to a baking dish and bake at 375°F for 15 minutes until the top is just beginning to set. Remove from heat and garnish with fresh parsley before serving warm with crusty bread or crudités.