Preheat the oven to 375 degrees F (190 degrees C). Spray a small casserole dish with Vegetable Oil Cooking Spray (to taste). In a large bowl, combine the Yukon Gold Potato (3 cup), Scallion (2), Egg (1), Milk (1/3 cup) and Blue Cheese (3 ounce).
Stir until well combined. Transfer the mashed potatoes to the casserole dish and smooth the top with the back of a spoon.
Place the Sourdough Bread (2 slice), Fresh Parsley (1/4 cup) and Butter (1 tablespoon) the bowl of a mini-prep food processor.
Pulse until fine breadcrumbs form and parsley is chopped.
Top the potato mixture with the breadcrumbs and sprinkle with Blue Cheese (1 tablespoon).
Bake for 20 to 25 minutes until topping is browned and bubbly.