In a small bowl, whisk together the butter and olive oil. Brush both sides of the baguette slices with mixture and line them up on a baking sheet. Set aside.
In another small bowl, stir together the cranberries, balsamic vinegar, sugar, and rosemary. Spread these out on a rimmed baking sheet. (They get pretty sticky, so if you want to avoid scrubbing, line the pan with parchment paper).
Place the cranberries in the oven. Roast for five minutes, then add the tray of baguette slices to the oven. Roast for an additional 6 minutes then remove the baguette slices, flip them over, and return to the oven for another 4-6 minutes, or until crispy and golden. At this point, the cranberries should be popped open with juices running out. Remove both the cranberries and baguette slices from the oven.
Top each baguette with a slice of brie and a spoonful of cranberries. Garnish with thyme if desired. These can be served while the cranberries are still warm, or you can make the components ahead of time. The cranberries can be chilled overnight and the crostini will keep in a zip lock bag. Simply assemble and serve at room temperature when you're ready!