Foam the butter in a large pan over medium-low heat. Add the onions, thyme leaves and a pinch of salt. Cook on low for up to 1 hour or until onions are soft and golden brown. Add balsamic vinegar, taste and season with additional salt and pepper. Set aside or cover and refrigerate until ready to use.
Preheat oven to 180ºC.
Place Castello Extra Creamy Brie in a Brie baker. Cover and bake for 10 minutes. Remove from oven and garnish with caramelized onions. Allow to rest for 5 minutes before serving.
Topping recommendations: blanched fresh asparagus spear slices, pickled plums, pears or raspberries with balsamic vinegar. Serve with toasted baguette slices and/or crackers.