Place the flour, milk, eggs, onion powder and salt in a blender or food processor. Blend for 10 seconds, scraping down the sides and then blend for another 10 seconds. Allow the batter to rest in the blender for 20 minutes.
Place a 9 or 10 inch oven safe skillet on t he middle rack of the oven and remove any racks above it. Heat the oven to 425.
Remove the skillet from the oven using oven mitts and place it on top of the stove.
Add the butter and allow it to melt, swirling to coat the bottom and sides of the pan. Pour the batter on top of the butter being sure it sits evenly across all sides of the pan. Place the skillet in the oven.
Bake the Dutch baby until it is puffed and lightly browned across the top, about 15 - 20 minutes.
Meanwhile, warm leftover mashed potatoes, turkey and cranberry sauce. Spoon over top and sprinkle with the peas. Drizzle gravy over top and serve.
