Preheat the oven to 400 degrees. In a bowl, toss the mushrooms with the balsamic vinegar, olive oil, rosemary, salt and pepper to taste.
Line a rimmed baking pan with foil and spread the mushrooms in the pan. Roast, tossing every 5 minutes, until the liquid has evaporated and the mushrooms are beginning to crisp in spots, about 25 minutes.
Meanwhile, lightly spray the baguette slices on both sides with olive oil and place them on a baking sheet. Toast in the oven until the tops are golden, about 4 minutes. Sprinkle each toast with 1/2 teaspoon Parmesan cheese and return to the oven until the cheese is melted.
Pile the mushrooms on the toasted baguette slices; serve.