Stir blackberry jam, mustard, and rosemary together in a small bowl.
Spread mixture on one slice of bread, layer with Brie, and top with second slice.
Coat a nonstick skillet with cooking spray and heat over medium-low.
Spray outer sides of bread, place in skillet, cover, and cook until golden on one side, about 3 minutes.
Flip, cover, and cook until golden and Brie is melted, 2–3 minutes more.
