Position a rack 6-inches from the top of the oven and then preheat the oven to 400°F.
Arrange the baguette slices on two large rimmed baking sheets. Brush all over with the olive oil. Bake, flipping once, until crisp, about 10 minutes.
Remove the crostini from the oven and turn the broiler on to HIGH.
Top each crostini with a slice of brie. One baking sheet at a time, place the crostini under the broiler and broil until the cheese is melty and the crostini is beginning to brown around the edges, about 1 minute. (Do not step away from your oven—this process will happen quickly!) Remove from the oven and repeat with the second baking sheet of crostini.
Top each crostini with a small ribbon of prosciutto and a dollop of the Balsamic Onion Jam. Transfer to large plates or a platter.
Drizzle with olive oil and garnish with fresh rosemary leaves. Sprinkle with flaky sea salt, if desired, and serve right away.
**NOTE: To get thin ¼-inch thick slices of brie, it helps to slice it cold when it’s right out of the fridge.**