Preheat the oven to 400F and line a rimmed baking sheet with parchment.
Wrap the brie in the prosciutto and brush with the oil.
Toss the pecans in a bowl with the garlic, rosemary, butter, and salt and pepper. Place the pecans on the Brie and bake 10 to 15 minutes (I found I had to bake mine longer to get it melty...could be the brand of cheese).
If you have too many pecans to top the Brie, simply place them around on the baking sheet to toast. But watch them carefully as they will cook faster than the brie. Simply remove when they are nicely toasted.