Melt butter in heavy large skillet over medium-high heat. Add all mushrooms and garlic and saute until mushrooms are cooked through and brown, about 10 minutes. Add cream and boil until liquid is completely absorbed, about 2 minutes. Remove from heat. Add blue cheese and stir until cheese melts. Mix in prosciutto. Season to taste with salt and pepper. Transfer mushroom topping to bowl. (Mushroom topping can be made 1 day ahead. Cover and refrigerate.) Preheat oven to 375°F. Arrange bread slices on baking sheet. Bake until just golden, about 5 minutes. Mound 1 generous tablespoon mushroom topping on each slice. Return to oven; bake until topping is heated through, about 6 minutes. Sprinkle with chopped parsley.