Melt butter in a heavy large skillet over medium high hear. Add all mushrooms and garlic and sauteed 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 bleu cheese and stir until cheese melts.
Mix in proscuitto. Season with salt and pepper. Transfer to a bowl. (Mushroom topping can be made 1 day ahead. Cover and refrigerate. If you make this the day before make sure to bring the topping mixture to room temperature so it is easy to work with.).
Preheat oven to 375 degrees. Arrange bread slices on baking sheet. Bake until just golden, about 5 minutes.
Mound 1 generous tablespoon mushroom topping on each slice of bread. Return to oven and bake until heated through, about 6 minutes.