Start by either slicing or cubing 6 good-sized potatoes.
Spray 9 x 13 inch dish with cooking spray. Place potatoes in dish.
Melt 1/2 cup of butter over medium heat. Add 1 teaspoon garlic powder, 1 teaspoon onion powder, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Whisk together.
Add 1/3 cup of flour. Whisk continually. Mixture will get thick.
Add 2 cups milk. Bring to boil while stirring continually. Mixture will begin to thicken.
Stir in 3/4 cup parmesan cheese. Cook and whisk until smooth.
Pour parmesan cheese mixture over potatoes, stir to coat. Bake for 30 minutes at 350 degrees.
After baking for 30 minutes, stir the potatoes and continue to bake for another 30 minutes.
After baking an additional 30 minutes, stir potatoes again, and sprinkle 1/4 cup parmesan cheese on the top. Bake for an additional 20 minutes.