PLACE oven rack in the lowest position; PREHEAT oven to 350°F; LIGHTLY mist a 9"-10" deep-dish pie plate or 8 x 8 deep baking dish with cooking spray.
MELT 3 tablespoons butter in a large microwave-safe bowl; ADD potatoes to butter; MIX well.
TRANSFER potato mixture into prepared baking dish; FIRMLY press the mixture in uniform thickness, on the bottom and up the sides of dish to form a crust.
SPRINKLE the parmesan cheese evenly onto inner crust area; PRESS parmesan cheese gently to create a seal; SMOOTH top of crust edge while running fingers over potato along the perimeter of baking dish, to create a basket-weave effect.
INTO the same large bowl, add the eggs, flour and Alfredo sauce; WHISK until smooth.
FOLD in 1 cup of the cheese, reserving 1/4 cup; ADD 4 tablespoons of the bacon (reserving 1 tablespoon), 1 tablespoon of the onions (reserving 1 tablespoon), 1 tablespoon white wine vinegar, and 1 pinch white pepper; FOLD to to combine.
POUR mixture into crust; SPRINKLE with reserved bacon, then onions.
PLACE on bottom oven rack; BAKE for 50-60 minutes, until middle is puffed and crust is golden brown, and center springs back lightly when touched.
SET on a cooling rack and let cool slightly before cutting.