Preheat oven to 400 degrees F.
Peel potatoes and place in a baking dish large enough to hold them. Add about 1" of water and cover the dish with foil. Bake potatoes for 40 minutes, or until tender through to center when pierced with a sharp knife. They should be tender enough to slice but not falling-apart tender. Remove from oven and use a slotted spoon to transfer potatoes to a cutting board. Allow to cool for 20 minutes or until cool enough to handle. Slice potatoes into 1" thick rounds and then quarter the rounds. Sprinkle with a little salt, to taste.
To make the bechamel, add butter to a Dutch oven and place over LOW heat. Allow butter to melt, being careful it does not brown. Add flour and cook, whisking, for about 3 to 4 minutes. Gradually whisk in the warm milk, a little at a time, whisking as you add it to smooth out the lumps. Continue whisking until completely smooth and nicely thickened. Remove from heat. Stir the salt and onion powder into the bechamel. Add cheese and stir until melted. Fold potatoes into cheesy bechamel and mix until potatoes are coated well with the sauce.
Transfer mixture to prepared baking dish that has been coated with non-stick cooking spray, cover with foil, and bake at 375 degrees for 40 minutes, or until hot and bubbly. Remove foil, set oven to BROIL, and cook for an additional 2 or 3 minutes, or until top is nicely browned. Alternately, remove foil during last 15 minutes of baking time for a more subtle browning.