Melt the butter in a small pot over medium-low heat. And the onion and cook, stirring, until golden and sticky, about 10 minutes. Remove from heat and cool to room temperature. Place baguette rounds on a baking sheet. Brush with olive oil. Broil until lightly toasted. Turnover and lightly toast the other side. Cut the cheese into thin slices just large enough to fit the baguette rounds. Broil until cheese just begins to melt. Arrange Crostini on a platter. Top each with spoon of the onion mixture, garnish with a rosemary sprig and serve.