peel and slice thin enough potatoes to measure about 6 cups, place sliced potatoes in a bowl of iced water so they will not brown while you make the cheese sauce
preheat oven to 325*F
in pan on the stove over med heat melt butter, stir in the onion powder, turn down heat to med-low and stir in flour and pepper, stirring constantly, until it is bubbly, remove from heat
stir in milk and 2&1/2 cups of shredded cheese, heat this to a boil over med-low to med heat stirring constantly, once it reaches a boil, boil and stir for 1 min, then take off heat
with non stick cooking spray, spray a 9x13baking dish, spread half the potatoes in the pan, then spread half of the remaining cheddar cheese over potatoes, now pour all of cheese sauce over potatoes, u might have to let cheese sauce work its way down, sprinkle with remaining cheese on top and bake uncovered in the oven at 325*F for 1&1/2 hrs to1 hr 45 min, when knife or fork is stuck in it should slide in easy, if top gets to brown, cover with foil, but try not to if u can