Melt the butter in a large skillet over medium heat. Stir in the onions and a big pinch each of salt and pepper, then add 1/2 cup broth. Gently simmer until the onions are deeply golden and caramelized, adding broth 1/3 cup at a time. This will take 30-40 minutes depending on your onions. Monitor the heat as you cook to ensure the onions don't burn.
In the meantime, preheat oven to 400 degrees. Brush both sides of your sliced bread with olive oil and bake 10-12 minute until golden.
When the onions are ready, make the crostini. Add a slice of brie to each bread piece and toast for 5-8 minutes depending on how gooey you want your brie. Then spoon a heap of onions overtop (about 1/4 cup).