Heat oven to 400 degrees. Spray a sheet pan with cooking spray. Place the destemmed mushrooms top side down on pan and sprinkle lightly with salt.In a large bowl combine the cream cheese, boursin cheese, chopped walnuts and bacon. Use a small spoon to fill the mushroom caps with the cheese mixture. Sprinkle the mushrooms with panko and drizzle with olive oil.Bake for 20 to 25 minutes. The mushrooms should be softened and the filling should be golden on top. While the mushrooms are baking, simmer the balsamic vinegar in a small saucepan over medium-low heat until it reduces to 1/4 cup. Set aside.Allow the mushrooms to rest for a few minutes then drizzle them with the reduced balsamic.