Melt the butter in a small skillet set over medium heat. Add the cubed mushroom, garlic and rosemary and cook for 3 to 4 minutes. Add the wine and cook until it has reduced by half; remove the skillet from the heat.
Preheat the oven to 400 degrees F. Place baguette slices on a parchment paper-lined baking sheet. Cut the cheese into thin slices just large enough to fit the top of the baguette slices. Top the cheese with the mushrooms.
Bake for 5 minutes, or until the cheese just begins to melt. Cool slightly, arrange on a plate and garnish each melt with a rosemary sprig before serving.